diff --git a/src/contracts/ERC20Safe.sol b/src/contracts/ERC20Safe.sol
index 4b5b02e2..7a0fc3de 100755
--- a/src/contracts/ERC20Safe.sol
+++ b/src/contracts/ERC20Safe.sol
@@ -12,6 +12,11 @@ import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Burnable.sol";
     @notice This contract is intended to be used with ERC20Handler contract.
  */
 contract ERC20Safe {
+
+    error ERC20NonContractCall();
+    error ERC20CallFailed();
+    error ERC20OperationFailed();
+
     /**
         @notice Used to gain custody of deposited token.
         @param tokenAddress Address of ERC20 to transfer.
@@ -88,13 +93,13 @@ contract ERC20Safe {
         assembly {
             tokenSize := extcodesize(token)
         }
-        require(tokenSize > 0, "ERC20: not a contract");
+        if (tokenSize <= 0) revert ERC20NonContractCall();
 
         (bool success, bytes memory returndata) = address(token).call(data);
-        require(success, "ERC20: call failed");
+        if (!success) revert ERC20CallFailed();
 
         if (returndata.length > 0) {
-            require(abi.decode(returndata, (bool)), "ERC20: operation did not succeed");
+            if (!abi.decode(returndata, (bool))) revert ERC20OperationFailed();
         }
     }
 }
diff --git a/src/contracts/Executor.sol b/src/contracts/Executor.sol
index 660e0b6b..e5ec36e1 100644
--- a/src/contracts/Executor.sol
+++ b/src/contracts/Executor.sol
@@ -48,6 +48,7 @@ contract Executor is Context {
     error EmptyProposalsArray();
     error BridgeIsPaused();
     error ZeroAddressProvided();
+    error NoVerifierAddressProvided();
     error AccessNotAllowed(address sender, bytes4 funcSig);
     error TransferHashDoesNotMatchSlotValue(bytes32 transferHash);
     error StateRootDoesNotMatch(IStateRootStorage stateRootStorage, bytes32 stateRoot);
@@ -107,7 +108,7 @@ contract Executor is Context {
         @param verifiersAddresses Array of verifiers addresses which store state roots.
      */
     function adminSetVerifiers(uint8 securityModel, address[] memory verifiersAddresses) external onlyAllowed {
-        require(verifiersAddresses.length > 0, "Should provide at least one verifier address");
+        if (verifiersAddresses.length <= 0) revert NoVerifierAddressProvided();
         _securityModels[securityModel] = verifiersAddresses;
     }
 
diff --git a/src/contracts/Router.sol b/src/contracts/Router.sol
index 1de14d11..12143649 100644
--- a/src/contracts/Router.sol
+++ b/src/contracts/Router.sol
@@ -34,6 +34,8 @@ contract Router is Context {
     error AccessNotAllowed(address sender, bytes4 funcSig);
     error BridgeIsPaused();
     error ZeroAddressProvided();
+    error NonceDecrementNotAllowed(uint64 currentNonce);
+    error MsgValueNotZero();
 
     event Deposit(
         uint8 destinationDomainID,
@@ -74,7 +76,8 @@ contract Router is Context {
         @param nonce The nonce value to be set.
      */
     function adminSetDepositNonce(uint8 domainID, uint64 nonce) external onlyAllowed {
-        require(nonce > _depositCounts[domainID], "Does not allow decrements of the nonce");
+        uint64 currentNonce = _depositCounts[domainID];
+        if (nonce <= currentNonce) revert NonceDecrementNotAllowed(currentNonce);
         _depositCounts[domainID] = nonce;
     }
 
@@ -103,7 +106,7 @@ contract Router is Context {
         if (handler == address(0)) revert ResourceIDNotMappedToHandler();
 
         if (address(feeHandler) == address(0)) {
-            require(msg.value == 0, "no FeeHandler, msg.value != 0");
+            if (msg.value != 0) revert MsgValueNotZero();
         } else {
             // Reverts on failure
             feeHandler.collectFee{value: msg.value}(
diff --git a/src/contracts/handlers/ERCHandlerHelpers.sol b/src/contracts/handlers/ERCHandlerHelpers.sol
index d6c622f1..def207e6 100755
--- a/src/contracts/handlers/ERCHandlerHelpers.sol
+++ b/src/contracts/handlers/ERCHandlerHelpers.sol
@@ -30,6 +30,7 @@ abstract contract ERCHandlerHelpers is IERCHandler {
 
     error ContractAddressNotWhitelisted(address contractAddress);
     error DepositAmountTooSmall(uint256 depositAmount);
+    error SenderNotBridgeRouterOrExecutor();
 
     // resourceID => token contract address
     mapping(bytes32 => address) public _resourceIDToTokenContractAddress;
diff --git a/src/contracts/handlers/FeeHandlerRouter.sol b/src/contracts/handlers/FeeHandlerRouter.sol
index 5e1cfa0c..727a71ea 100755
--- a/src/contracts/handlers/FeeHandlerRouter.sol
+++ b/src/contracts/handlers/FeeHandlerRouter.sol
@@ -24,6 +24,8 @@ contract FeeHandlerRouter is IFeeHandler, AccessControl {
     event WhitelistChanged(address whitelistAddress, bool isWhitelisted);
 
     error IncorrectFeeSupplied(uint256);
+    error SenderNotRouterContract();
+    error SenderNotAdmin();
 
     modifier onlyRouter() {
         _onlyRouter();
@@ -31,7 +33,7 @@ contract FeeHandlerRouter is IFeeHandler, AccessControl {
     }
 
     function _onlyRouter() private view {
-        require(msg.sender == _routerAddress, "sender must be router contract");
+        if (msg.sender != _routerAddress) revert SenderNotRouterContract();
     }
 
     modifier onlyAdmin() {
@@ -40,7 +42,7 @@ contract FeeHandlerRouter is IFeeHandler, AccessControl {
     }
 
     function _onlyAdmin() private view {
-        require(hasRole(DEFAULT_ADMIN_ROLE, _msgSender()), "sender doesn't have admin role");
+        if (!hasRole(DEFAULT_ADMIN_ROLE, _msgSender())) revert SenderNotAdmin();
     }
 
     /**
diff --git a/src/contracts/handlers/PermissionlessGenericHandler.sol b/src/contracts/handlers/PermissionlessGenericHandler.sol
index f2770745..cf966e37 100755
--- a/src/contracts/handlers/PermissionlessGenericHandler.sol
+++ b/src/contracts/handlers/PermissionlessGenericHandler.sol
@@ -20,8 +20,14 @@ contract PermissionlessGenericHandler is IHandler {
         _;
     }
 
+    error SenderNotBridgeContract();
+    error SenderNotExecutorContract();
+    error IncorrectDataLength(uint256 dataLength);
+    error RequestedFeeTooLarge(uint256 maxFee);
+    error InvalidExecutioDataDepositor(address executioDataDepositor);
+
     function _onlyBridge() private view {
-        require(msg.sender == _bridgeAddress, "sender must be bridge contract");
+        if (msg.sender != _bridgeAddress) revert SenderNotBridgeContract();
     }
 
     modifier onlyExecutor() {
@@ -30,7 +36,7 @@ contract PermissionlessGenericHandler is IHandler {
     }
 
     function _onlyExecutor() private view {
-        require(msg.sender == _executorAddress, "sender must be executor contract");
+        if (msg.sender != _executorAddress) revert SenderNotExecutorContract();
     }
 
     /**
@@ -113,7 +119,7 @@ contract PermissionlessGenericHandler is IHandler {
             executeFuncSignature(address executionDataDepositor, uint256[] uintArray, address addr)
      */
     function deposit(bytes32 resourceID, address depositor, bytes calldata data) external pure returns (bytes memory) {
-        require(data.length >= 76, "Incorrect data length"); // 32 + 2 + 1 + 1 + 20 + 20
+        if (data.length < 76) revert IncorrectDataLength(data.length); // 32 + 2 + 1 + 1 + 20 + 20
 
         uint256 maxFee;
         uint16 lenExecuteFuncSignature;
@@ -142,8 +148,8 @@ contract PermissionlessGenericHandler is IHandler {
             )
         );
 
-        require(maxFee < MAX_FEE, "requested fee too large");
-        require(depositor == executionDataDepositor, "incorrect depositor in deposit data");
+        if (maxFee > MAX_FEE) revert RequestedFeeTooLarge(maxFee);
+        if (depositor != executionDataDepositor) revert InvalidExecutioDataDepositor(executionDataDepositor);
         return data;
     }
 
diff --git a/src/contracts/handlers/fee/BasicFeeHandler.sol b/src/contracts/handlers/fee/BasicFeeHandler.sol
index 1ebe8455..d5b6033a 100755
--- a/src/contracts/handlers/fee/BasicFeeHandler.sol
+++ b/src/contracts/handlers/fee/BasicFeeHandler.sol
@@ -22,26 +22,32 @@ contract BasicFeeHandler is IFeeHandler, AccessControl {
 
     event FeeChanged(uint256 newFee);
 
+    error SenderNotBridgeOrRouter();
     error IncorrectFeeSupplied(uint256);
     error ZeroAddressProvided();
+    error SenderNotAdmin();
+    error CannotRenounceOneself();
+    error NewFeeEqualsCurrentFee(uint256 currentFee);
+    error AddressesAndAmountsArraysDifferentLength(
+        uint256 addressesLength,
+        uint256 amountsLength
+    );
+    error EtherFeeTransferFailed();
 
     modifier onlyAdmin() {
-        require(hasRole(DEFAULT_ADMIN_ROLE, msg.sender), "sender doesn't have admin role");
+        if (!hasRole(DEFAULT_ADMIN_ROLE, msg.sender)) revert SenderNotAdmin();
         _;
     }
 
-    modifier onlyBridgeOrRouter() {
-        _onlyBridgeOrRouter();
+    modifier onlyRouterOrFeeRouter() {
+        _onlyRouterOrFeeRouter();
         _;
     }
 
-    function _onlyBridgeOrRouter() private view {
-        require(
-            msg.sender == _bridgeAddress ||
-            msg.sender == _feeHandlerRouterAddress ||
-            msg.sender == _routerAddress,
-            "sender must be bridge or fee router contract"
-        );
+    function _onlyRouterOrFeeRouter() private view {
+        if (msg.sender != _feeHandlerRouterAddress &&
+            msg.sender != _routerAddress
+        ) revert SenderNotBridgeOrRouter();
     }
 
     /**
@@ -67,7 +73,7 @@ contract BasicFeeHandler is IFeeHandler, AccessControl {
      */
     function renounceAdmin(address newAdmin) external {
         address sender = _msgSender();
-        require(sender != newAdmin, "Cannot renounce oneself");
+        if (sender == newAdmin) revert CannotRenounceOneself();
         grantRole(DEFAULT_ADMIN_ROLE, newAdmin);
         renounceRole(DEFAULT_ADMIN_ROLE, sender);
     }
@@ -90,7 +96,7 @@ contract BasicFeeHandler is IFeeHandler, AccessControl {
         uint8 securityModel,
         bytes calldata depositData,
         bytes calldata feeData
-    ) external virtual payable onlyBridgeOrRouter {
+    ) external virtual payable onlyRouterOrFeeRouter {
         uint256 currentFee = _domainResourceIDSecurityModelToFee[destinationDomainID][resourceID][securityModel];
         if (msg.value != currentFee) revert IncorrectFeeSupplied(msg.value);
         emit FeeCollected(sender, fromDomainID, destinationDomainID, resourceID, currentFee, address(0));
@@ -135,7 +141,7 @@ contract BasicFeeHandler is IFeeHandler, AccessControl {
             uint256 newFee
         ) external onlyAdmin {
         uint256 currentFee = _domainResourceIDSecurityModelToFee[destinationDomainID][resourceID][securityModel];
-        require(currentFee != newFee, "Current fee is equal to new fee");
+        if (currentFee == newFee) revert NewFeeEqualsCurrentFee(currentFee);
         _domainResourceIDSecurityModelToFee[destinationDomainID][resourceID][securityModel] = newFee;
         emit FeeChanged(newFee);
     }
@@ -148,10 +154,13 @@ contract BasicFeeHandler is IFeeHandler, AccessControl {
         @param amounts Array of amounts to transfer to {addrs}.
      */
     function transferFee(address payable[] calldata addrs, uint256[] calldata amounts) external onlyAdmin {
-        require(addrs.length == amounts.length, "addrs[], amounts[]: diff length");
+        if (addrs.length != amounts.length) revert AddressesAndAmountsArraysDifferentLength(
+            addrs.length,
+            amounts.length
+        );
         for (uint256 i = 0; i < addrs.length; i++) {
             (bool success, ) = addrs[i].call{value: amounts[i]}("");
-            require(success, "Fee ether transfer failed");
+            if (!success) revert EtherFeeTransferFailed();
             emit FeeDistributed(address(0), addrs[i], amounts[i]);
         }
     }
diff --git a/src/contracts/handlers/fee/PercentageERC20FeeHandlerEVM.sol b/src/contracts/handlers/fee/PercentageERC20FeeHandlerEVM.sol
index 298a265e..0d4d0a4a 100755
--- a/src/contracts/handlers/fee/PercentageERC20FeeHandlerEVM.sol
+++ b/src/contracts/handlers/fee/PercentageERC20FeeHandlerEVM.sol
@@ -29,6 +29,10 @@ contract PercentageERC20FeeHandlerEVM is BasicFeeHandler, ERC20Safe {
 
     event FeeBoundsChanged(uint256 newLowerBound, uint256 newUpperBound);
 
+    error MsgValueNotZero();
+    error InvalidBoundsRatio(uint128 newLowerBound, uint128 newUpperBound);
+    error NewBoundsEqualCurrentBounds(uint128 currentLowerBound, uint128 currentUpperBound);
+
     /**
         @param bridgeAddress Contract address of previously deployed Bridge.
         @param feeHandlerRouterAddress Contract address of previously deployed FeeHandlerRouter.
@@ -121,8 +125,8 @@ contract PercentageERC20FeeHandlerEVM is BasicFeeHandler, ERC20Safe {
         uint8 securityModel,
         bytes calldata depositData,
         bytes calldata feeData
-    ) external payable override onlyBridgeOrRouter {
-        require(msg.value == 0, "collectFee: msg.value != 0");
+    ) external payable override onlyRouterOrFeeRouter {
+        if (msg.value != 0) revert MsgValueNotZero();
 
         (uint256 fee, address tokenAddress) = _calculateFee(
             sender,
@@ -146,15 +150,14 @@ contract PercentageERC20FeeHandlerEVM is BasicFeeHandler, ERC20Safe {
         @param newUpperBound Value {_newUpperBound} will be updated to.
      */
     function changeFeeBounds(bytes32 resourceID, uint128 newLowerBound, uint128 newUpperBound) external onlyAdmin {
-        require(
-            newUpperBound == 0 || (newUpperBound > newLowerBound),
-            "Upper bound must be larger than lower bound or 0"
-        );
+        if (newUpperBound != 0 && (newUpperBound <= newLowerBound)) {
+            revert InvalidBoundsRatio(newLowerBound, newUpperBound);
+        }
+
         Bounds memory existingBounds = _resourceIDToFeeBounds[resourceID];
-        require(
-            existingBounds.lowerBound != newLowerBound || existingBounds.upperBound != newUpperBound,
-            "Current bounds are equal to new bounds"
-        );
+        if (existingBounds.lowerBound == newLowerBound && existingBounds.upperBound == newUpperBound) {
+            revert NewBoundsEqualCurrentBounds(existingBounds.lowerBound, existingBounds.upperBound);
+        }
 
         Bounds memory newBounds = Bounds(newLowerBound, newUpperBound);
         _resourceIDToFeeBounds[resourceID] = newBounds;
@@ -175,7 +178,10 @@ contract PercentageERC20FeeHandlerEVM is BasicFeeHandler, ERC20Safe {
         address[] calldata addrs,
         uint256[] calldata amounts
     ) external onlyAdmin {
-        require(addrs.length == amounts.length, "addrs[], amounts[]: diff length");
+        if (addrs.length != amounts.length) revert AddressesAndAmountsArraysDifferentLength(
+            addrs.length,
+            amounts.length
+        );
         address tokenHandler = IBridge(_bridgeAddress)._resourceIDToHandlerAddress(resourceID);
         address tokenAddress = IERCHandler(tokenHandler)._resourceIDToTokenContractAddress(resourceID);
         for (uint256 i = 0; i < addrs.length; i++) {
diff --git a/src/contracts/libraries/Bytes.sol b/src/contracts/libraries/Bytes.sol
index 2c1b0198..da649aa3 100644
--- a/src/contracts/libraries/Bytes.sol
+++ b/src/contracts/libraries/Bytes.sol
@@ -7,6 +7,10 @@ pragma solidity ^0.8.11;
  * @notice Bytes is a library for manipulating byte arrays.
  */
 library Bytes {
+
+    error SliceOwerflow();
+    error SliceOutOfBounds();
+
     /**
      * @notice Slices a byte array with a given starting index and length. Returns a new byte array
      *         as opposed to a pointer to the original array. Will throw if trying to slice more
@@ -24,9 +28,9 @@ library Bytes {
         uint256 _length
     ) internal pure returns (bytes memory) {
         unchecked {
-            require(_length + 31 >= _length, "slice_overflow");
-            require(_start + _length >= _start, "slice_overflow");
-            require(_bytes.length >= _start + _length, "slice_outOfBounds");
+            if (_length + 31 < _length) revert SliceOwerflow();
+            if (_start + _length < _start) revert SliceOwerflow();
+            if (_bytes.length < _start + _length) revert SliceOutOfBounds();
         }
 
         bytes memory tempBytes;
diff --git a/src/contracts/libraries/MerkleTrie.sol b/src/contracts/libraries/MerkleTrie.sol
index 8b333abe..9c522d9b 100644
--- a/src/contracts/libraries/MerkleTrie.sol
+++ b/src/contracts/libraries/MerkleTrie.sol
@@ -21,6 +21,19 @@ library MerkleTrie {
         RLPReader.RLPItem[] decoded;
     }
 
+    error MerkleTrieEmptyKey();
+    error MerkleTrieKeyIndexExceedsKeyLength();
+    error MerkleTrieInvalidRootHash();
+    error MerkleTrieInvalidInternalNodeHash();
+    error MerkleTrieInvalidLargeInternalHash();
+    error MerkleTrieValueNodeNotLastInProof();
+    error RLPReaderDecodedItemIsNotAListItem();
+    error MerkleTriePathMustShareAllNibblesWithKey();
+    error MerkleTrieKeyRemainderMustEqualPathRemainder();
+    error MerkleTrieValueLengthIsZero();
+    error MerkleTrieNodeWithUnknownPrefix();
+    error MerkleTrieUnparsableNode();
+
     /// @notice Determines the number of elements per branch node.
     uint256 internal constant TREE_RADIX = 16;
 
@@ -70,7 +83,7 @@ library MerkleTrie {
     /// @param _root  Known root of the Merkle trie.
     /// @return value_ Value of the key if it exists.
     function get(bytes memory _key, bytes[] memory _proof, bytes32 _root) internal pure returns (bytes memory value_) {
-        require(_key.length > 0, "MerkleTrie: empty key");
+        if (_key.length <= 0) revert MerkleTrieEmptyKey();
 
         TrieNode[] memory proof = _parseProof(_proof);
         bytes memory key = Bytes.toNibbles(_key);
@@ -82,23 +95,23 @@ library MerkleTrie {
             TrieNode memory currentNode = proof[i];
 
             // Key index should never exceed total key length or we'll be out of bounds.
-            require(currentKeyIndex <= key.length, "MerkleTrie: key index exceeds total key length");
+            if (currentKeyIndex > key.length) revert MerkleTrieKeyIndexExceedsKeyLength();
 
             if (currentKeyIndex == 0) {
                 // First proof element is always the root node.
-                require(
-                    Bytes.equal(abi.encodePacked(keccak256(currentNode.encoded)), currentNodeID),
-                    "MerkleTrie: invalid root hash"
-                );
+                if (!Bytes.equal(abi.encodePacked(keccak256(currentNode.encoded)), currentNodeID)) {
+                    revert MerkleTrieInvalidRootHash();
+                }
             } else if (currentNode.encoded.length >= 32) {
                 // Nodes 32 bytes or larger are hashed inside branch nodes.
-                require(
-                    Bytes.equal(abi.encodePacked(keccak256(currentNode.encoded)), currentNodeID),
-                    "MerkleTrie: invalid large internal hash"
-                );
+                if (!Bytes.equal(abi.encodePacked(keccak256(currentNode.encoded)), currentNodeID)) {
+                    revert MerkleTrieInvalidLargeInternalHash();
+                }
             } else {
                 // Nodes smaller than 32 bytes aren't hashed.
-                require(Bytes.equal(currentNode.encoded, currentNodeID), "MerkleTrie: invalid internal node hash");
+                if (!Bytes.equal(currentNode.encoded, currentNodeID)) {
+                    revert MerkleTrieInvalidInternalNodeHash();
+                }
             }
 
             if (currentNode.decoded.length == BRANCH_NODE_LENGTH) {
@@ -109,10 +122,10 @@ library MerkleTrie {
                     // even when the value wasn't explicitly placed there. Geth treats a value of
                     // bytes(0) as "key does not exist" and so we do the same.
                     value_ = RLPReader.readBytes(currentNode.decoded[TREE_RADIX]);
-                    require(value_.length > 0, "MerkleTrie: value length must be greater than zero (branch)");
+                    if (value_.length <= 0) revert MerkleTrieValueLengthIsZero();
 
                     // Extra proof elements are not allowed.
-                    require(i == proof.length - 1, "MerkleTrie: value node must be last node in proof (branch)");
+                    if (i != proof.length - 1) revert MerkleTrieValueNodeNotLastInProof();
 
                     return value_;
                 } else {
@@ -134,10 +147,7 @@ library MerkleTrie {
                 // Whether this is a leaf node or an extension node, the path remainder MUST be a
                 // prefix of the key remainder (or be equal to the key remainder) or the proof is
                 // considered invalid.
-                require(
-                    pathRemainder.length == sharedNibbleLength,
-                    "MerkleTrie: path remainder must share all nibbles with key"
-                );
+                if (pathRemainder.length != sharedNibbleLength) revert MerkleTriePathMustShareAllNibblesWithKey();
 
                 if (prefix == PREFIX_LEAF_EVEN || prefix == PREFIX_LEAF_ODD) {
                     // Prefix of 2 or 3 means this is a leaf node. For the leaf node to be valid,
@@ -146,20 +156,19 @@ library MerkleTrie {
                     // the key remainder length equals the shared nibble length, which implies
                     // equality with the path remainder (since we already did the same check with
                     // the path remainder and the shared nibble length).
-                    require(
-                        keyRemainder.length == sharedNibbleLength,
-                        "MerkleTrie: key remainder must be identical to path remainder"
-                    );
+                    if (keyRemainder.length != sharedNibbleLength) {
+                        revert MerkleTrieKeyRemainderMustEqualPathRemainder();
+                    }
 
                     // Our Merkle Trie is designed specifically for the purposes of the Ethereum
                     // state trie. Empty values are not allowed in the state trie, so we can safely
                     // say that if the value is empty, the key should not exist and the proof is
                     // invalid.
                     value_ = RLPReader.readBytes(currentNode.decoded[1]);
-                    require(value_.length > 0, "MerkleTrie: value length must be greater than zero (leaf)");
+                    if (value_.length <= 0) revert MerkleTrieValueLengthIsZero();
 
                     // Extra proof elements are not allowed.
-                    require(i == proof.length - 1, "MerkleTrie: value node must be last node in proof (leaf)");
+                    if (i != proof.length - 1) revert MerkleTrieValueNodeNotLastInProof();
 
                     return value_;
                 } else if (prefix == PREFIX_EXTENSION_EVEN || prefix == PREFIX_EXTENSION_ODD) {
@@ -169,10 +178,10 @@ library MerkleTrie {
                     currentNodeID = _getNodeID(currentNode.decoded[1]);
                     currentKeyIndex += sharedNibbleLength;
                 } else {
-                    revert("MerkleTrie: received a node with an unknown prefix");
+                    revert MerkleTrieNodeWithUnknownPrefix();
                 }
             } else {
-                revert("MerkleTrie: received an unparseable node");
+                revert MerkleTrieUnparsableNode();
             }
         }
 
diff --git a/src/contracts/libraries/RLPReader.sol b/src/contracts/libraries/RLPReader.sol
index 2e915014..03b57569 100644
--- a/src/contracts/libraries/RLPReader.sol
+++ b/src/contracts/libraries/RLPReader.sol
@@ -41,6 +41,18 @@ library RLPReader {
      */
     uint256 internal constant MAX_LIST_LENGTH = 32;
 
+    error RLPReaderDecodedItemIsNotAListItem();
+    error RLPReaderListItemHasInvalidDataRemainder();
+    error RLPReaderInvalidContentLength();
+    error RLPItemTotalLengthGreaterThanContentLength();
+    error RLPReaderListLenghtGreaterThanContentLength();
+    error RLPReaderInvalidLeadingZeros();
+    error RLPReaderItemLengthIsZero();
+    error RLPReaderInvalidPrefix();
+    error RLPItemStringLengthGreaterThanContentLength();
+    error RLPReaderInvalidBytes32Value();
+    error RLPReaderInvalidAddressValue();
+
     /**
      * @notice Converts bytes to a reference to memory position and length.
      *
@@ -50,10 +62,7 @@ library RLPReader {
      */
     function toRLPItem(bytes memory _in) internal pure returns (RLPItem memory) {
         // Empty arrays are not RLP items.
-        require(
-            _in.length > 0,
-            "RLPReader: length of an RLP item must be greater than zero to be decodable"
-        );
+        if ( _in.length <= 0) revert RLPReaderItemLengthIsZero();
 
         MemoryPointer ptr;
         assembly {
@@ -73,15 +82,9 @@ library RLPReader {
     function readList(RLPItem memory _in) internal pure returns (RLPItem[] memory) {
         (uint256 listOffset, uint256 listLength, RLPItemType itemType) = _decodeLength(_in);
 
-        require(
-            itemType == RLPItemType.LIST_ITEM,
-            "RLPReader: decoded item type for list is not a list item"
-        );
+        if (itemType != RLPItemType.LIST_ITEM) revert RLPReaderDecodedItemIsNotAListItem();
 
-        require(
-            listOffset + listLength == _in.length,
-            "RLPReader: list item has an invalid data remainder"
-        );
+        if (listOffset + listLength != _in.length) revert RLPReaderListItemHasInvalidDataRemainder();
 
         // Solidity in-memory arrays can't be increased in size, but *can* be decreased in size by
         // writing to the length. Since we can't know the number of RLP items without looping over
@@ -139,15 +142,9 @@ library RLPReader {
     function readBytes(RLPItem memory _in) internal pure returns (bytes memory) {
         (uint256 itemOffset, uint256 itemLength, RLPItemType itemType) = _decodeLength(_in);
 
-        require(
-            itemType == RLPItemType.DATA_ITEM,
-            "RLPReader: decoded item type for bytes is not a data item"
-        );
+        if (itemType != RLPItemType.DATA_ITEM) revert RLPReaderDecodedItemIsNotAListItem();
 
-        require(
-            _in.length == itemOffset + itemLength,
-            "RLPReader: bytes value contains an invalid remainder"
-        );
+        if (_in.length != itemOffset + itemLength) revert RLPReaderListItemHasInvalidDataRemainder();
 
         return _copy(_in.ptr, itemOffset, itemLength);
     }
@@ -195,10 +192,7 @@ library RLPReader {
         // Short-circuit if there's nothing to decode, note that we perform this check when
         // the user creates an RLP item via toRLPItem, but it's always possible for them to bypass
         // that function and create an RLP item directly. So we need to check this anyway.
-        require(
-            _in.length > 0,
-            "RLPReader: length of an RLP item must be greater than zero to be decodable"
-        );
+        if (_in.length <= 0) revert RLPReaderItemLengthIsZero();
 
         MemoryPointer ptr = _in.ptr;
         uint256 prefix;
@@ -215,55 +209,37 @@ library RLPReader {
             // slither-disable-next-line variable-scope
             uint256 strLen = prefix - 0x80;
 
-            require(
-                _in.length > strLen,
-                "RLPReader: length of content must be greater than string length (short string)"
-            );
+            if (_in.length <= strLen) revert RLPItemStringLengthGreaterThanContentLength();
 
             bytes1 firstByteOfContent;
             assembly {
                 firstByteOfContent := and(mload(add(ptr, 1)), shl(248, 0xff))
             }
 
-            require(
-                strLen != 1 || firstByteOfContent >= 0x80,
-                "RLPReader: invalid prefix, single byte < 0x80 are not prefixed (short string)"
-            );
+            if (strLen == 1 && firstByteOfContent < 0x80) revert RLPReaderInvalidPrefix();
 
             return (1, strLen, RLPItemType.DATA_ITEM);
         } else if (prefix <= 0xbf) {
             // Long string.
             uint256 lenOfStrLen = prefix - 0xb7;
 
-            require(
-                _in.length > lenOfStrLen,
-                "RLPReader: length of content must be > than length of string length (long string)"
-            );
+            if (_in.length <= lenOfStrLen) revert RLPItemStringLengthGreaterThanContentLength();
 
             bytes1 firstByteOfContent;
             assembly {
                 firstByteOfContent := and(mload(add(ptr, 1)), shl(248, 0xff))
             }
 
-            require(
-                firstByteOfContent != 0x00,
-                "RLPReader: length of content must not have any leading zeros (long string)"
-            );
+            if (firstByteOfContent == 0x00) revert RLPReaderInvalidLeadingZeros();
 
             uint256 strLen;
             assembly {
                 strLen := shr(sub(256, mul(8, lenOfStrLen)), mload(add(ptr, 1)))
             }
 
-            require(
-                strLen > 55,
-                "RLPReader: length of content must be greater than 55 bytes (long string)"
-            );
+            if (strLen <= 55) revert RLPReaderInvalidContentLength();
 
-            require(
-                _in.length > lenOfStrLen + strLen,
-                "RLPReader: length of content must be greater than total length (long string)"
-            );
+            if (_in.length <= lenOfStrLen + strLen) revert RLPItemTotalLengthGreaterThanContentLength();
 
             return (1 + lenOfStrLen, strLen, RLPItemType.DATA_ITEM);
         } else if (prefix <= 0xf7) {
@@ -271,45 +247,30 @@ library RLPReader {
             // slither-disable-next-line variable-scope
             uint256 listLen = prefix - 0xc0;
 
-            require(
-                _in.length > listLen,
-                "RLPReader: length of content must be greater than list length (short list)"
-            );
+            if (_in.length <= listLen) revert RLPReaderListLenghtGreaterThanContentLength();
 
             return (1, listLen, RLPItemType.LIST_ITEM);
         } else {
             // Long list.
             uint256 lenOfListLen = prefix - 0xf7;
 
-            require(
-                _in.length > lenOfListLen,
-                "RLPReader: length of content must be > than length of list length (long list)"
-            );
+            if (_in.length <= lenOfListLen) revert RLPReaderListLenghtGreaterThanContentLength();
 
             bytes1 firstByteOfContent;
             assembly {
                 firstByteOfContent := and(mload(add(ptr, 1)), shl(248, 0xff))
             }
 
-            require(
-                firstByteOfContent != 0x00,
-                "RLPReader: length of content must not have any leading zeros (long list)"
-            );
+            if (firstByteOfContent == 0x00) revert RLPReaderInvalidLeadingZeros();
 
             uint256 listLen;
             assembly {
                 listLen := shr(sub(256, mul(8, lenOfListLen)), mload(add(ptr, 1)))
             }
 
-            require(
-                listLen > 55,
-                "RLPReader: length of content must be greater than 55 bytes (long list)"
-            );
+            if (listLen <= 55) revert RLPReaderInvalidContentLength();
 
-            require(
-                _in.length > lenOfListLen + listLen,
-                "RLPReader: length of content must be greater than total length (long list)"
-            );
+            if (_in.length <= lenOfListLen + listLen) revert RLPItemTotalLengthGreaterThanContentLength();
 
             return (1 + lenOfListLen, listLen, RLPItemType.LIST_ITEM);
         }
@@ -363,11 +324,11 @@ library RLPReader {
      * @return Decoded bytes32.
      */
     function readBytes32(RLPItem memory _in) internal pure returns (bytes32) {
-        require(_in.length <= 33, "Invalid RLP bytes32 value.");
+        if (_in.length > 33) revert RLPReaderInvalidBytes32Value();
 
         (uint256 itemOffset, uint256 itemLength, RLPItemType itemType) = _decodeLength(_in);
 
-        require(itemType == RLPItemType.DATA_ITEM, "Invalid RLP bytes32 value.");
+        if (itemType != RLPItemType.DATA_ITEM) revert RLPReaderInvalidBytes32Value();
 
         uint256 ptr = MemoryPointer.unwrap(_in.ptr) + itemOffset;
         bytes32 out;
@@ -402,7 +363,7 @@ library RLPReader {
             return address(0);
         }
 
-        require(_in.length == 21, "Invalid RLP address value.");
+        if (_in.length != 21) revert RLPReaderInvalidAddressValue();
 
         return address(uint160(readUint256(_in)));
     }
diff --git a/src/contracts/libraries/StorageProof.sol b/src/contracts/libraries/StorageProof.sol
index 9682149b..a91b389a 100644
--- a/src/contracts/libraries/StorageProof.sol
+++ b/src/contracts/libraries/StorageProof.sol
@@ -8,13 +8,17 @@ library StorageProof {
     using RLPReader for RLPReader.RLPItem;
     using RLPReader for bytes;
 
+    error StorageValueDoesNotExist();
+    error AccountDoesNotExist();
+    error InvalidAccountListLength();
+
     function getStorageValue(bytes32 slotHash, bytes32 storageRoot, bytes[] memory stateProof)
         internal pure
         returns (bytes32)
     {
         bytes memory valueRlpBytes =
             MerkleTrie.get(abi.encodePacked(slotHash), stateProof, storageRoot);
-        require(valueRlpBytes.length > 0, "Storage value does not exist");
+        if (valueRlpBytes.length <= 0) revert StorageValueDoesNotExist();
         return valueRlpBytes.toRLPItem().readBytes32();
     }
 
@@ -24,9 +28,9 @@ library StorageProof {
     {
         bytes32 addressHash = keccak256(abi.encodePacked(contractAddress));
         bytes memory acctRlpBytes = MerkleTrie.get(abi.encodePacked(addressHash), proof, stateRoot);
-        require(acctRlpBytes.length > 0, "Account does not exist");
+        if (acctRlpBytes.length <= 0) revert AccountDoesNotExist();
         RLPReader.RLPItem[] memory acctFields = acctRlpBytes.toRLPItem().readList();
-        require(acctFields.length == 4);
+        if (acctFields.length != 4) revert InvalidAccountListLength();
         return bytes32(acctFields[2].readUint256());
     }
 }
diff --git a/src/contracts/proxies/SpectreProxy.sol b/src/contracts/proxies/SpectreProxy.sol
index 7689e0c9..f404096d 100644
--- a/src/contracts/proxies/SpectreProxy.sol
+++ b/src/contracts/proxies/SpectreProxy.sol
@@ -25,13 +25,18 @@ contract SpectreProxy is AccessControl {
     event CommitteeRotated(uint8 sourceDomainID, uint256 slot);
     event StateRootSubmitted(uint8 sourceDomainID, uint256 slot, bytes32 stateRoot);
 
+    error SenderNotAdmin();
+    error ArrayLengthsDoNotMatch();
+    error SpectreAddressNotFound();
+    error InvalidMerkleProof();
+
     modifier onlyAdmin() {
         _onlyAdmin();
         _;
     }
 
     function _onlyAdmin() private view {
-        require(hasRole(DEFAULT_ADMIN_ROLE, _msgSender()), "sender doesn't have admin role");
+        if (!hasRole(DEFAULT_ADMIN_ROLE, _msgSender())) revert SenderNotAdmin();
     }
 
     /**
@@ -41,7 +46,7 @@ contract SpectreProxy is AccessControl {
         @param spectreAddresses List of spectre addresses.
     */
     constructor(uint8[] memory domainIDS, address[] memory spectreAddresses) {
-        require(domainIDS.length == spectreAddresses.length, "array length should be equal");
+        if (domainIDS.length != spectreAddresses.length) revert ArrayLengthsDoNotMatch();
         for (uint8 i = 0; i < domainIDS.length; i++) {
             spectreContracts[domainIDS[i]] = spectreAddresses[i];
         }
@@ -72,7 +77,7 @@ contract SpectreProxy is AccessControl {
         bytes calldata stepProof
     ) external {
         address spectreAddress = spectreContracts[sourceDomainID];
-        require(spectreAddress != address(0), "no spectre address found");
+        if (spectreAddress == address(0)) revert SpectreAddressNotFound();
 
         ISpectre spectre = ISpectre(spectreAddress);
         spectre.rotate(rotateProof, stepInput, stepProof);
@@ -95,16 +100,15 @@ contract SpectreProxy is AccessControl {
         bytes[] calldata stateRootProof
     ) external {
         address spectreAddress = spectreContracts[sourceDomainID];
-        require(spectreAddress != address(0), "no spectre address found");
+        if (spectreAddress == address(0)) revert SpectreAddressNotFound();
 
         ISpectre spectre = ISpectre(spectreAddress);
         spectre.step(input, stepProof);
 
         bytes32 executionRoot = spectre.executionPayloadRoots(input.finalizedSlot);
-        require(
-            verifyMerkleBranch(stateRoot, executionRoot, stateRootProof, STATE_ROOT_INDEX),
-            "invalid merkle proof"
-        );
+        if (!verifyMerkleBranch(stateRoot, executionRoot, stateRootProof, STATE_ROOT_INDEX)) {
+            revert InvalidMerkleProof();
+        }
 
         stateRoots[sourceDomainID][input.finalizedSlot] = stateRoot;
         emit StateRootSubmitted(sourceDomainID, input.finalizedSlot, stateRoot);
diff --git a/src/contracts/utils/AccessControlSegregator.sol b/src/contracts/utils/AccessControlSegregator.sol
index fd2e7324..181845fb 100755
--- a/src/contracts/utils/AccessControlSegregator.sol
+++ b/src/contracts/utils/AccessControlSegregator.sol
@@ -13,6 +13,9 @@ contract AccessControlSegregator {
 
     bytes4 public constant GRANT_ACCESS_SIG = AccessControlSegregator(address(0)).grantAccess.selector;
 
+    error ArrayLengthsDoNotMatch();
+    error SenderWithoutAccessRights();
+
     /**
         @notice Initializes access control to functions and sets initial
         access to grantAccess function.
@@ -20,7 +23,7 @@ contract AccessControlSegregator {
         @param accounts List of accounts.
     */
     constructor(bytes4[] memory functions, address[] memory accounts) {
-        require(accounts.length == functions.length, "array length should be equal");
+        if (accounts.length != functions.length) revert ArrayLengthsDoNotMatch();
 
         _grantAccess(GRANT_ACCESS_SIG, msg.sender);
         for (uint8 i = 0; i < accounts.length; i++) {
@@ -45,7 +48,7 @@ contract AccessControlSegregator {
         @param account Address of account.
     */
     function grantAccess(bytes4 sig, address account) public {
-        require(hasAccess(GRANT_ACCESS_SIG, msg.sender), "sender doesn't have grant access rights");
+        if (!hasAccess(GRANT_ACCESS_SIG, msg.sender)) revert SenderWithoutAccessRights();
 
         _grantAccess(sig, account);
     }
diff --git a/src/contracts/utils/Pausable.sol b/src/contracts/utils/Pausable.sol
index b66f1fa9..467380a5 100755
--- a/src/contracts/utils/Pausable.sol
+++ b/src/contracts/utils/Pausable.sol
@@ -11,6 +11,10 @@ pragma solidity 0.8.11;
  *
  */
 contract Pausable {
+
+    error ContractIsPaused();
+    error ContractIsNotPaused();
+
     /**
      * @dev Emitted when the pause is triggered by `account`.
      */
@@ -50,7 +54,7 @@ contract Pausable {
     }
 
     function _whenNotPaused() private view {
-        require(!_paused, "Pausable: paused");
+        if (_paused) revert ContractIsPaused();
     }
 
     /**
@@ -66,7 +70,7 @@ contract Pausable {
     }
 
     function _whenPaused() private view {
-        require(_paused, "Pausable: not paused");
+        if (!_paused) revert ContractIsNotPaused();
     }
 
     /**
diff --git a/test/contractBridge/admin.test.ts b/test/contractBridge/admin.test.ts
index ae165d4f..4f493c04 100644
--- a/test/contractBridge/admin.test.ts
+++ b/test/contractBridge/admin.test.ts
@@ -328,7 +328,10 @@ describe("Bridge - [admin]", () => {
       routerInstance
         .connect(tokenOwnerAccount)
         .adminSetDepositNonce(domainID, newNonce),
-    ).to.be.revertedWith("Does not allow decrements of the nonce");
+    ).to.be.revertedWithCustomError(
+      routerInstance,
+      "NonceDecrementNotAllowed(uint64)",
+    );
   });
 
   it("Should require admin role to mark nonce as used", async () => {
diff --git a/test/contractBridge/depositERC20.test.ts b/test/contractBridge/depositERC20.test.ts
index 4d6d7406..8163610d 100755
--- a/test/contractBridge/depositERC20.test.ts
+++ b/test/contractBridge/depositERC20.test.ts
@@ -9,6 +9,7 @@ import type {
   Depositor,
   ERC20Handler,
   ERC20PresetMinterPauser,
+  ERC20Safe__factory,
   Executor,
 } from "../../typechain-types";
 import {
@@ -35,6 +36,7 @@ describe("Bridge - [deposit - ERC20]", () => {
   let ERC20MintableInstance1: ERC20PresetMinterPauser;
   let ERC20MintableInstance2: ERC20PresetMinterPauser;
   let ERC20HandlerInstance: ERC20Handler;
+  let erc20SafeContract: ERC20Safe__factory;
   let depositorAccount: HardhatEthersSigner;
   let recipientAccount: HardhatEthersSigner;
 
@@ -111,6 +113,8 @@ describe("Bridge - [deposit - ERC20]", () => {
       20,
       await recipientAccount.getAddress(),
     );
+
+    erc20SafeContract = await ethers.getContractFactory("ERC20Safe");
   });
 
   it("[sanity] test depositorAccount' balance", async () => {
@@ -284,6 +288,6 @@ describe("Bridge - [deposit - ERC20]", () => {
           depositData,
           feeData,
         ),
-    ).to.be.revertedWith("ERC20: operation did not succeed");
+    ).to.be.revertedWithCustomError(erc20SafeContract, "ERC20OperationFailed");
   });
 });
diff --git a/test/contractBridge/executeProposalERC20.test.ts b/test/contractBridge/executeProposalERC20.test.ts
index 734691a0..a7b4e49c 100755
--- a/test/contractBridge/executeProposalERC20.test.ts
+++ b/test/contractBridge/executeProposalERC20.test.ts
@@ -15,6 +15,7 @@ import type {
   ERC20Handler,
   ERC20PresetMinterPauser,
   StateRootStorage,
+  MerkleTrie__factory,
 } from "../../typechain-types";
 
 describe("Bridge - [execute proposal - ERC20]", () => {
@@ -38,6 +39,7 @@ describe("Bridge - [execute proposal - ERC20]", () => {
   let ERC20MintableInstance: ERC20PresetMinterPauser;
   let ERC20HandlerInstance: ERC20Handler;
   let stateRootStorageInstance: StateRootStorage;
+  let MerkleTrieContract: MerkleTrie__factory;
   let depositorAccount: HardhatEthersSigner;
   let recipientAccount: HardhatEthersSigner;
   let relayer1: HardhatEthersSigner;
@@ -126,6 +128,8 @@ describe("Bridge - [execute proposal - ERC20]", () => {
       slot,
       stateRoot,
     );
+
+    MerkleTrieContract = await ethers.getContractFactory("MerkleTrie");
   });
 
   it("isProposalExecuted returns false if depositNonce is not used", async () => {
@@ -299,8 +303,9 @@ describe("Bridge - [execute proposal - ERC20]", () => {
       .connect(relayer1)
       .executeProposal(invalidProposal, accountProof1, slot);
 
-    await expect(proposalTx).to.be.revertedWith(
-      "MerkleTrie: invalid large internal hash",
+    await expect(proposalTx).to.be.revertedWithCustomError(
+      MerkleTrieContract,
+      "MerkleTrieInvalidLargeInternalHash",
     );
 
     // check that deposit nonce has not been marked as used in bitmap
@@ -347,7 +352,10 @@ describe("Bridge - [execute proposal - ERC20]", () => {
     it("[sanity] should fail if verifiers array is empty", async () => {
       await expect(
         executorInstance.adminSetVerifiers(1, []),
-      ).to.be.rejectedWith("Should provide at least one verifier address");
+      ).to.be.revertedWithCustomError(
+        executorInstance,
+        "NoVerifierAddressProvided",
+      );
     });
 
     it("should successfully execute a proposal", async () => {
diff --git a/test/handlers/erc20/deposit.test.ts b/test/handlers/erc20/deposit.test.ts
index b669983e..9e41b261 100755
--- a/test/handlers/erc20/deposit.test.ts
+++ b/test/handlers/erc20/deposit.test.ts
@@ -15,6 +15,7 @@ import type {
   Executor,
   ERC20Handler,
   ERC20PresetMinterPauser,
+  ERC20Safe__factory,
 } from "../../../typechain-types";
 
 describe("ERC20Handler - [Deposit ERC20]", () => {
@@ -32,6 +33,7 @@ describe("ERC20Handler - [Deposit ERC20]", () => {
   let executorInstance: Executor;
   let ERC20MintableInstance: ERC20PresetMinterPauser;
   let ERC20HandlerInstance: ERC20Handler;
+  let erc20SafeContract: ERC20Safe__factory;
   let adminAccount: HardhatEthersSigner;
   let depositorAccount: HardhatEthersSigner;
 
@@ -73,6 +75,8 @@ describe("ERC20Handler - [Deposit ERC20]", () => {
         emptySetResourceData,
       ),
     ]);
+
+    erc20SafeContract = await ethers.getContractFactory("ERC20Safe");
   });
 
   it("[sanity] depositor owns tokenAmount of ERC20", async () => {
@@ -203,7 +207,7 @@ describe("ERC20Handler - [Deposit ERC20]", () => {
           ),
           feeData,
         ),
-    ).to.be.revertedWith("ERC20: not a contract");
+    ).to.be.revertedWithCustomError(erc20SafeContract, "ERC20NonContractCall");
 
     await expect(
       routerInstance
@@ -219,6 +223,6 @@ describe("ERC20Handler - [Deposit ERC20]", () => {
           ),
           feeData,
         ),
-    ).to.be.revertedWith("ERC20: not a contract");
+    ).to.be.revertedWithCustomError(erc20SafeContract, "ERC20NonContractCall");
   });
 });
diff --git a/test/handlers/fee/basic/admin.test.ts b/test/handlers/fee/basic/admin.test.ts
index 22754e39..c1bbfc8a 100755
--- a/test/handlers/fee/basic/admin.test.ts
+++ b/test/handlers/fee/basic/admin.test.ts
@@ -149,7 +149,7 @@ describe("BasicFeeHandler - [admin]", () => {
       basicFeeHandlerInstance
         .connect(nonAdminAccount)
         .changeFee(destinationDomainID, resourceID, securityModel, fee),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(basicFeeHandlerInstance, "SenderNotAdmin");
   });
 
   it("BasicFeeHandler admin should be changed to newBasicFeeHandlerAdmin", async () => {
diff --git a/test/handlers/fee/basic/changeFee.test.ts b/test/handlers/fee/basic/changeFee.test.ts
index 246a6963..b7af0e81 100755
--- a/test/handlers/fee/basic/changeFee.test.ts
+++ b/test/handlers/fee/basic/changeFee.test.ts
@@ -106,7 +106,10 @@ describe("BasicFeeHandler - [changeFee]", () => {
         securityModel,
         0,
       ),
-    ).to.be.rejectedWith("Current fee is equal to new fee");
+    ).to.be.revertedWithCustomError(
+      basicFeeHandlerInstance,
+      "NewFeeEqualsCurrentFee",
+    );
   });
 
   it("should require admin role to change fee", async () => {
@@ -114,6 +117,6 @@ describe("BasicFeeHandler - [changeFee]", () => {
       basicFeeHandlerInstance
         .connect(nonAdminAccount)
         .changeFee(destinationDomainID, resourceID, securityModel, 1),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(basicFeeHandlerInstance, "SenderNotAdmin");
   });
 });
diff --git a/test/handlers/fee/basic/collectFee.test.ts b/test/handlers/fee/basic/collectFee.test.ts
index a223dd5c..df2cfbd3 100755
--- a/test/handlers/fee/basic/collectFee.test.ts
+++ b/test/handlers/fee/basic/collectFee.test.ts
@@ -246,7 +246,7 @@ describe("BasicFeeHandler - [collectFee]", () => {
             value: ethers.parseEther("1.0"),
           },
         ),
-    ).to.be.rejectedWith("no FeeHandler, msg.value != 0");
+    ).to.be.revertedWithCustomError(routerInstance, "MsgValueNotZero");
   });
 
   it("deposit should pass if fee handler not set and fee not supplied", async () => {
@@ -314,7 +314,10 @@ describe("BasicFeeHandler - [collectFee]", () => {
             value: ethers.parseEther("0.5"),
           },
         ),
-    ).to.be.revertedWith("sender must be bridge or fee router contract");
+    ).to.be.revertedWithCustomError(
+      basicFeeHandlerInstance,
+      "SenderNotBridgeOrRouter",
+    );
 
     const balanceAfter = await ethers.provider.getBalance(
       await basicFeeHandlerInstance.getAddress(),
@@ -373,7 +376,10 @@ describe("BasicFeeHandler - [collectFee]", () => {
             value: ethers.parseEther("0.5"),
           },
         ),
-    ).to.be.revertedWith("sender must be router contract");
+    ).to.be.revertedWithCustomError(
+      feeHandlerRouterInstance,
+      "SenderNotRouterContract",
+    );
 
     const balanceAfter = await ethers.provider.getBalance(
       await basicFeeHandlerInstance.getAddress(),
diff --git a/test/handlers/fee/basic/distributeFee.test.ts b/test/handlers/fee/basic/distributeFee.test.ts
index b78e1730..937683d6 100755
--- a/test/handlers/fee/basic/distributeFee.test.ts
+++ b/test/handlers/fee/basic/distributeFee.test.ts
@@ -243,7 +243,7 @@ describe("BasicFeeHandler - [distributeFee]", () => {
           ],
           [payout, payout],
         ),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(basicFeeHandlerInstance, "SenderNotAdmin");
   });
 
   it("should revert if addrs and amounts arrays have different length", async () => {
@@ -283,6 +283,9 @@ describe("BasicFeeHandler - [distributeFee]", () => {
         [await recipientAccount1.getAddress(), recipientAccount2.getAddress()],
         [payout, payout, payout],
       ),
-    ).to.be.revertedWith("addrs[], amounts[]: diff length");
+    ).to.be.revertedWithCustomError(
+      basicFeeHandlerInstance,
+      "AddressesAndAmountsArraysDifferentLength",
+    );
   });
 });
diff --git a/test/handlers/fee/handlerRouter.test.ts b/test/handlers/fee/handlerRouter.test.ts
index 9bf1b641..0e794d05 100755
--- a/test/handlers/fee/handlerRouter.test.ts
+++ b/test/handlers/fee/handlerRouter.test.ts
@@ -108,7 +108,7 @@ describe("FeeHandlerRouter", () => {
           securityModel,
           feeHandlerAccount.getAddress(),
         ),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(feeHandlerRouterInstance, "SenderNotAdmin");
   });
 
   it("should successfully set whitelist on an address", async () => {
@@ -139,7 +139,7 @@ describe("FeeHandlerRouter", () => {
       feeHandlerRouterInstance
         .connect(nonAdminAccount)
         .adminSetWhitelist(await nonWhitelistedAccount.getAddress(), true),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(feeHandlerRouterInstance, "SenderNotAdmin");
   });
 
   it("should return fee 0 if address whitelisted", async () => {
diff --git a/test/handlers/fee/percentage/admin.test.ts b/test/handlers/fee/percentage/admin.test.ts
index aeefc08d..353371d2 100755
--- a/test/handlers/fee/percentage/admin.test.ts
+++ b/test/handlers/fee/percentage/admin.test.ts
@@ -140,7 +140,10 @@ describe("PercentageFeeHandler - [admin]", () => {
       percentageFeeHandlerInstance
         .connect(nonAdminAccount)
         .changeFee(destinationDomainID, resourceID, securityModel, fee),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "SenderNotAdmin",
+    );
   });
 
   it("should set fee bounds", async () => {
@@ -182,7 +185,10 @@ describe("PercentageFeeHandler - [admin]", () => {
       percentageFeeHandlerInstance
         .connect(nonAdminAccount)
         .changeFeeBounds(resourceID, lowerBound, upperBound),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "SenderNotAdmin",
+    );
   });
 
   it("PercentageFeeHandler admin should be changed to newPercentageFeeHandlerAdmin", async () => {
diff --git a/test/handlers/fee/percentage/changeFee.test.ts b/test/handlers/fee/percentage/changeFee.test.ts
index 210fa075..d8f153a0 100755
--- a/test/handlers/fee/percentage/changeFee.test.ts
+++ b/test/handlers/fee/percentage/changeFee.test.ts
@@ -93,7 +93,10 @@ describe("PercentageFeeHandler - [change fee and bounds]", () => {
         securityModel,
         0,
       ),
-    ).to.be.revertedWith("Current fee is equal to new fee");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "NewFeeEqualsCurrentFee",
+    );
   });
 
   it("should require admin role to change fee", async () => {
@@ -101,7 +104,10 @@ describe("PercentageFeeHandler - [change fee and bounds]", () => {
       percentageFeeHandlerInstance
         .connect(nonAdminAddress)
         .changeFee(destinationDomainID, resourceID, securityModel, 1),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "SenderNotAdmin",
+    );
   });
 
   it("should set fee bounds", async () => {
@@ -131,10 +137,13 @@ describe("PercentageFeeHandler - [change fee and bounds]", () => {
     await percentageFeeHandlerInstance.changeFeeBounds(resourceID, 25, 50);
     await expect(
       percentageFeeHandlerInstance.changeFeeBounds(resourceID, 25, 50),
-    ).to.be.revertedWith("Current bounds are equal to new bounds");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "NewBoundsEqualCurrentBounds",
+    );
   });
 
-  it("should fail to set lower bound larger than upper bound ", async () => {
+  it("should fail to set lower bound larger than upper bound", async () => {
     const percentageFeeHandlerInstance =
       await PercentageFeeHandlerContract.deploy(
         await bridgeInstance.getAddress(),
@@ -143,7 +152,10 @@ describe("PercentageFeeHandler - [change fee and bounds]", () => {
       );
     await expect(
       percentageFeeHandlerInstance.changeFeeBounds(resourceID, 50, 25),
-    ).to.be.revertedWith("Upper bound must be larger than lower bound or 0");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "InvalidBoundsRatio",
+    );
   });
 
   it("should set only lower bound", async () => {
@@ -197,6 +209,9 @@ describe("PercentageFeeHandler - [change fee and bounds]", () => {
       percentageFeeHandlerInstance
         .connect(nonAdminAddress)
         .changeFeeBounds(resourceID, 50, 100),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "SenderNotAdmin",
+    );
   });
 });
diff --git a/test/handlers/fee/percentage/collectFee.test.ts b/test/handlers/fee/percentage/collectFee.test.ts
index 57fa7b8f..3e9b39c9 100755
--- a/test/handlers/fee/percentage/collectFee.test.ts
+++ b/test/handlers/fee/percentage/collectFee.test.ts
@@ -185,7 +185,10 @@ describe("PercentageFeeHandler - [collectFee]", () => {
             value: ethers.parseEther("0.5"),
           },
         ),
-    ).to.be.revertedWith("collectFee: msg.value != 0");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "MsgValueNotZero",
+    );
   });
 
   it("deposit should revert if fee collection fails", async () => {
@@ -238,7 +241,10 @@ describe("PercentageFeeHandler - [collectFee]", () => {
             value: ethers.parseEther("0.5").toString(),
           },
         ),
-    ).to.be.revertedWith("sender must be bridge or fee router contract");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "SenderNotBridgeOrRouter",
+    );
   });
 
   it("deposit should revert if not called by bridge on FeeHandlerRouter contract", async () => {
@@ -266,7 +272,10 @@ describe("PercentageFeeHandler - [collectFee]", () => {
             value: ethers.parseEther("0.5"),
           },
         ),
-    ).to.be.revertedWith("sender must be router contract");
+    ).to.be.revertedWithCustomError(
+      feeHandlerRouterInstance,
+      "SenderNotRouterContract",
+    );
   });
 
   it("should successfully change fee handler from FeeRouter to PercentageFeeHandler and collect fee", async () => {
diff --git a/test/handlers/fee/percentage/distributeFee.test.ts b/test/handlers/fee/percentage/distributeFee.test.ts
index 3ba391f3..b9eca9ff 100644
--- a/test/handlers/fee/percentage/distributeFee.test.ts
+++ b/test/handlers/fee/percentage/distributeFee.test.ts
@@ -256,7 +256,10 @@ describe("PercentageFeeHandler - [distributeFee]", () => {
           ],
           [payout, payout],
         ),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "SenderNotAdmin",
+    );
   });
 
   it("should revert if addrs and amounts arrays have different length", async () => {
@@ -285,6 +288,9 @@ describe("PercentageFeeHandler - [distributeFee]", () => {
         ],
         [payout, payout, payout],
       ),
-    ).to.be.revertedWith("addrs[], amounts[]: diff length");
+    ).to.be.revertedWithCustomError(
+      percentageFeeHandlerInstance,
+      "AddressesAndAmountsArraysDifferentLength",
+    );
   });
 });
diff --git a/test/handlers/generic/permissionlessDeposit.test.ts b/test/handlers/generic/permissionlessDeposit.test.ts
index f354088a..11bd2f60 100755
--- a/test/handlers/generic/permissionlessDeposit.test.ts
+++ b/test/handlers/generic/permissionlessDeposit.test.ts
@@ -131,7 +131,10 @@ describe("PermissionlessGenericHandler - [deposit]", () => {
           invalidDepositData,
           feeData,
         ),
-    ).to.be.revertedWith("Incorrect data length");
+    ).to.be.revertedWithCustomError(
+      permissionlessGenericHandlerInstance,
+      "IncorrectDataLength",
+    );
   });
 
   it("should revert if metadata encoded depositor does not match deposit depositor", async () => {
@@ -153,7 +156,10 @@ describe("PermissionlessGenericHandler - [deposit]", () => {
           invalidDepositData,
           feeData,
         ),
-    ).to.be.revertedWith("incorrect depositor in deposit data");
+    ).to.be.revertedWithCustomError(
+      permissionlessGenericHandlerInstance,
+      "InvalidExecutioDataDepositor",
+    );
   });
 
   it("should revert if max fee exceeds 1000000", async () => {
@@ -179,6 +185,9 @@ describe("PermissionlessGenericHandler - [deposit]", () => {
             from: depositorAccount,
           },
         ),
-    ).to.be.revertedWith("requested fee too large");
+    ).to.be.revertedWithCustomError(
+      permissionlessGenericHandlerInstance,
+      "RequestedFeeTooLarge",
+    );
   });
 });
diff --git a/test/proxies/spectre/spectreProxy.test.ts b/test/proxies/spectre/spectreProxy.test.ts
index 33266c78..e2700cbc 100644
--- a/test/proxies/spectre/spectreProxy.test.ts
+++ b/test/proxies/spectre/spectreProxy.test.ts
@@ -80,7 +80,7 @@ describe("Spectre Proxy", () => {
       spectreProxyInstance
         .connect(nonAdminAccount)
         .adminSetSpectreAddress(originDomainID, spectreAddress),
-    ).to.be.revertedWith("sender doesn't have admin role");
+    ).to.be.revertedWithCustomError(spectreProxyInstance, "SenderNotAdmin");
   });
 
   it("should set spectre address with an admin role", async () => {
@@ -103,7 +103,10 @@ describe("Spectre Proxy", () => {
         stepInput,
         stepProof,
       ),
-    ).to.be.revertedWith("no spectre address found");
+    ).to.be.revertedWithCustomError(
+      spectreProxyInstance,
+      "SpectreAddressNotFound",
+    );
   });
 
   it("should emit event even if rotate successful", async () => {
@@ -128,7 +131,10 @@ describe("Spectre Proxy", () => {
         validStateRoot,
         validStateRootProof,
       ),
-    ).to.be.revertedWith("no spectre address found");
+    ).to.be.revertedWithCustomError(
+      spectreProxyInstance,
+      "SpectreAddressNotFound",
+    );
   });
 
   it("should revert if step proof not valid", async () => {
@@ -140,7 +146,7 @@ describe("Spectre Proxy", () => {
         validStateRoot,
         invalidStateRootProof,
       ),
-    ).to.be.revertedWith("invalid merkle proof");
+    ).to.be.revertedWithCustomError(spectreProxyInstance, "InvalidMerkleProof");
   });
 
   it("should revert if step state root not valid", async () => {
@@ -152,7 +158,7 @@ describe("Spectre Proxy", () => {
         invalidStateRoot,
         validStateRootProof,
       ),
-    ).to.be.revertedWith("invalid merkle proof");
+    ).to.be.revertedWithCustomError(spectreProxyInstance, "InvalidMerkleProof");
   });
 
   it("should emit event and store state root if step valid", async () => {
diff --git a/test/utils/accessControlSegregator/constructor.test.ts b/test/utils/accessControlSegregator/constructor.test.ts
index c733c95f..127811da 100755
--- a/test/utils/accessControlSegregator/constructor.test.ts
+++ b/test/utils/accessControlSegregator/constructor.test.ts
@@ -68,7 +68,10 @@ describe("AccessControlSegregator - [constructor]", () => {
         ["0xa973ec93", "0x78728c73"],
         [adminAccount],
       ),
-    ).to.be.revertedWith("array length should be equal");
+    ).to.be.revertedWithCustomError(
+      accessControlSegregatorInstance,
+      "ArrayLengthsDoNotMatch",
+    );
   });
 
   it("should grant deployer grant access rights", async () => {
diff --git a/test/utils/accessControlSegregator/grantAccess.test.ts b/test/utils/accessControlSegregator/grantAccess.test.ts
index f6763af1..256178d7 100755
--- a/test/utils/accessControlSegregator/grantAccess.test.ts
+++ b/test/utils/accessControlSegregator/grantAccess.test.ts
@@ -39,7 +39,10 @@ describe("AccessControlSegregator - [grant access]", () => {
       accessControlSegregatorInstance
         .connect(accountWithoutAccess)
         .grantAccess(functionSignature, receivingAccessAccount),
-    ).to.be.revertedWith("sender doesn't have grant access rights");
+    ).to.be.revertedWithCustomError(
+      accessControlSegregatorInstance,
+      "SenderWithoutAccessRights",
+    );
   });
 
   it("should successfully grant access to a function", async () => {