diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index e962a28d..7d27c7ce 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -18,6 +18,10 @@ jobs:
         bundler-cache: true
     - name: Run RuboCop
       run: bundle exec rubocop
+    - name: Run Steep / RBS Type signature verification
+      run: |
+        bundle exec steep check --steepfile=./Steepfile --severity-level=error | ruby -pe 'sub(/^(.+):(\d+):(\d+): (.+)$/, %q{::error file=\1,line=\2,col=\3::\4})'
+      shell: bash
 
   test:
     runs-on: ${{ matrix.os }}-latest
diff --git a/Steepfile b/Steepfile
new file mode 100644
index 00000000..88ceffb9
--- /dev/null
+++ b/Steepfile
@@ -0,0 +1,35 @@
+D = Steep::Diagnostic
+
+target :lib do
+  signature "*.rbs"
+
+  # NOTE: All client-exposed methods/types should now have type signatures / rbs support
+  check "lib/unleash/client.rb"
+  check "lib/unleash/context.rb"
+  check "lib/unleash/variant.rb"
+  check "lib/unleash/scheduled_executor.rb"
+
+  # TODO: add signatures to the rest
+  # Mostly internal SDK files.
+  ignore "lib/unleash/bootstrap"
+  ignore "lib/unleash/strategy/*.rb"
+  ignore "lib/unleash/util"
+
+  ignore "lib/unleash/constraint.rb"
+  ignore "lib/unleash/feature_toggle.rb"
+  ignore "lib/unleash/metrics.rb"
+  ignore "lib/unleash/metrics_reporter.rb"
+  ignore "lib/unleash/toggle_fetcher.rb"
+  ignore "lib/unleash/variant_definition.rb"
+  ignore "lib/unleash/variant_override.rb"
+
+  # library "pathname", "set"       # Standard libraries
+  # library "strong_json"           # Gems
+
+  # configure_code_diagnostics(D::Ruby.strict)       # `strict` diagnostics setting
+  # configure_code_diagnostics(D::Ruby.lenient)      # `lenient` diagnostics setting
+  configure_code_diagnostics do |hash|             # You can setup everything yourself
+    hash[D::Ruby::NoMethod] = :information
+    hash[D::Ruby::UnsupportedSyntax] = :information
+  end
+end
diff --git a/lib/unleash/client.rb b/lib/unleash/client.rb
index c88024fe..2abbbacd 100644
--- a/lib/unleash/client.rb
+++ b/lib/unleash/client.rb
@@ -162,7 +162,7 @@ def register
     end
 
     def disabled_variant
-      @disabled_variant ||= Unleash::FeatureToggle.disabled_variant
+      Unleash::FeatureToggle.disabled_variant
     end
 
     def first_fetch_is_eager
diff --git a/lib/unleash/client.rbs b/lib/unleash/client.rbs
new file mode 100644
index 00000000..b3e17d9f
--- /dev/null
+++ b/lib/unleash/client.rbs
@@ -0,0 +1,39 @@
+module Unleash
+  class Client
+    attr_accessor fetcher_scheduled_executor: ScheduledExecutor
+
+    attr_accessor metrics_scheduled_executor: ScheduledExecutor
+
+    def initialize: (*untyped opts) -> void
+
+    def is_enabled?: (untyped feature, ?Context? context, ?bool default_value_param) ?{ () -> bool } -> bool
+
+    # enabled? is a more ruby idiomatic method name than is_enabled?
+    alias enabled? is_enabled?
+
+    # execute a code block (passed as a parameter), if is_enabled? is true.
+    def if_enabled: (untyped feature, ?Context context, ?bool default_value) { (untyped) -> bool } -> (untyped | nil)
+
+    def get_variant: (untyped feature, ?Context context, ?Variant fallback_variant) -> Variant
+
+    # safe shutdown: also flush metrics to server and toggles to disk
+    def shutdown: () -> nil
+
+    # quick shutdown: just kill running threads
+    def shutdown!: () -> nil
+
+    private
+
+    def info: () -> { appName: String, instanceId: String, sdkVersion: String, strategies: Array[String], started: String, interval: Integer }
+
+    def start_toggle_fetcher: () -> nil
+
+    def start_metrics: () -> nil
+
+    def register: () -> nil
+
+    def disabled_variant: () -> Variant
+
+    def first_fetch_is_eager: () -> bool
+  end
+end
diff --git a/lib/unleash/context.rbs b/lib/unleash/context.rbs
new file mode 100644
index 00000000..3c544d73
--- /dev/null
+++ b/lib/unleash/context.rbs
@@ -0,0 +1,21 @@
+module Unleash
+  class Context
+    ATTRS: ::Array[:app_name | :environment | :user_id | :session_id | :remote_address | :current_time]
+
+    def initialize: (?::Hash[String, any] params) -> Context
+
+    def to_s: () -> ::String
+
+    def get_by_name: ((String | Symbol) name) -> any
+
+    def include?: ((String | Symbol) name) -> bool
+
+    private
+
+    # Method to fetch values from hash for two types of keys: string in camelCase and symbol in snake_case
+    def value_for: ((String | Symbol) key, Hash[String, any] params, ?any? default_value) -> any
+
+    # converts CamelCase to snake_case
+    def underscore: ((String | Symbol) camel_cased_word) -> String
+  end
+end
diff --git a/lib/unleash/scheduled_executor.rbs b/lib/unleash/scheduled_executor.rbs
new file mode 100644
index 00000000..6751642f
--- /dev/null
+++ b/lib/unleash/scheduled_executor.rbs
@@ -0,0 +1,29 @@
+module Unleash
+  class ScheduledExecutor
+    attr_accessor name: String
+
+    attr_accessor interval: Numeric
+
+    attr_accessor max_exceptions: ::Integer
+
+    attr_accessor retry_count: ::Integer
+
+    attr_accessor thread: (Thread | nil)
+
+    attr_accessor immediate_execution: bool
+
+    def initialize: (String name, untyped interval, ?::Integer max_exceptions, ?bool immediate_execution) -> void
+
+    def run: () ?{ () -> nil } -> nil
+
+    def running?: () -> bool
+
+    def exit: () -> nil
+
+    private
+
+    def run_blk: () { () -> nil } -> nil
+
+    def exceeded_max_exceptions?: () -> bool
+  end
+end
diff --git a/lib/unleash/variant.rbs b/lib/unleash/variant.rbs
new file mode 100644
index 00000000..43486f0b
--- /dev/null
+++ b/lib/unleash/variant.rbs
@@ -0,0 +1,15 @@
+module Unleash
+  class Variant
+    attr_accessor name: String
+
+    attr_accessor enabled: bool
+
+    attr_accessor payload: String
+
+    def initialize: (?::Hash[String, String] params) -> void
+
+    def to_s: () -> ::String
+
+    def ==: (Variant other) -> bool
+  end
+end
diff --git a/unleash-client.gemspec b/unleash-client.gemspec
index ec156341..3b53a9b5 100644
--- a/unleash-client.gemspec
+++ b/unleash-client.gemspec
@@ -39,4 +39,12 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "simplecov", "~> 0.21.2"
   spec.add_development_dependency "simplecov-lcov", "~> 0.8.0"
+
+  # NOTE: only require rbs/steep in supported ruby versions. In EOL ruby/jruby, just ignore.
+  # rubocop:disable Gemspec/RubyVersionGlobalsUsage
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('2.6') && RUBY_ENGINE != 'jruby'
+    spec.add_development_dependency "rbs", "~> 2.8"
+    spec.add_development_dependency "steep", "~> 1.3"
+  end
+  # rubocop:enable Gemspec/RubyVersionGlobalsUsage
 end