diff --git a/src/main/scala/com/raquo/airstream/core/BaseObservable.scala b/src/main/scala/com/raquo/airstream/core/BaseObservable.scala
index 9f869e74..23d8d811 100644
--- a/src/main/scala/com/raquo/airstream/core/BaseObservable.scala
+++ b/src/main/scala/com/raquo/airstream/core/BaseObservable.scala
@@ -1,7 +1,7 @@
 package com.raquo.airstream.core
 
 import com.raquo.airstream.debug.Debugger
-import com.raquo.airstream.flatten.FlattenStrategy
+import com.raquo.airstream.flatten.{AllowFlatMap, FlattenStrategy, MergingStrategy, SwitchingStrategy}
 import com.raquo.airstream.ownership.{Owner, Subscription}
 import com.raquo.ew.JsArray
 
@@ -53,11 +53,44 @@ trait BaseObservable[+Self[+_] <: Observable[_], +A] extends Source[A] with Name
 
   def mapToUnit: Self[Unit] = map(_ => ())
 
-  /** @param compose Note: guarded against exceptions */
-  @inline def flatMap[B, Inner[_], Output[+_] <: Observable[_]](compose: A => Inner[B])(
-    implicit strategy: FlattenStrategy[Self, Inner, Output]
+  /** #WARNING: DO NOT USE THIS METHOD.
+    * See https://github.com/raquo/Airstream/#flattening-observables
+    */
+  @inline def flatMap[B, Inner[_], Output[+_] <: Observable[_]](
+    project: A => Inner[B]
+  )(
+    implicit strategy: SwitchingStrategy[Self, Inner, Output],
+    allowFlatMap: AllowFlatMap
+  ): Output[B] = {
+    strategy.flatten(map(project))
+  }
+
+  /** Alias to flatMapSwitch(_ => s) */
+  @inline def flatMapTo[B, Inner[_], Output[+_] <: Observable[_]](s: Inner[B])(
+    implicit strategy: SwitchingStrategy[Self, Inner, Output]
+  ): Output[B] = {
+    strategy.flatten(map(_ => s))
+  }
+
+  /** @param project Note: guarded against exceptions */
+  @inline def flatMapSwitch[B, Inner[_], Output[+_] <: Observable[_]](project: A => Inner[B])(
+    implicit strategy: SwitchingStrategy[Self, Inner, Output]
+  ): Output[B] = {
+    strategy.flatten(map(project))
+  }
+
+  /** @param project Note: guarded against exceptions */
+  @inline def flatMapMerge[B, Inner[_], Output[+_] <: Observable[_]](project: A => Inner[B])(
+    implicit strategy: MergingStrategy[Self, Inner, Output]
+  ): Output[B] = {
+    strategy.flatten(map(project))
+  }
+
+  /** @param project Note: guarded against exceptions */
+  @inline def flatMapCustom[B, Inner[_], Output[+_] <: Observable[_]](project: A => Inner[B])(
+    strategy: FlattenStrategy[Self, Inner, Output]
   ): Output[B] = {
-    strategy.flatten(map(compose))
+    strategy.flatten(map(project))
   }
 
   /** Distinct events (but keep all errors) by == (equals) comparison */
diff --git a/src/main/scala/com/raquo/airstream/core/Observable.scala b/src/main/scala/com/raquo/airstream/core/Observable.scala
index c6cbd465..9694caea 100644
--- a/src/main/scala/com/raquo/airstream/core/Observable.scala
+++ b/src/main/scala/com/raquo/airstream/core/Observable.scala
@@ -1,8 +1,8 @@
 package com.raquo.airstream.core
 
 import com.raquo.airstream.debug.DebuggableObservable
-import com.raquo.airstream.flatten.FlattenStrategy
 import com.raquo.airstream.flatten.FlattenStrategy._
+import com.raquo.airstream.flatten.{AllowFlatten, FlattenStrategy, MergingStrategy, SwitchingStrategy}
 
 // @TODO[Scala3] Put this trait together with BaseObservable in the same file, and make BaseObservable sealed.
 
@@ -23,20 +23,41 @@ object Observable extends ObservableLowPriorityImplicits {
   ) extends AnyVal {
 
     @inline def flatten[Output[+_] <: Observable[_]](
-      implicit strategy: FlattenStrategy[Outer, Inner, Output]
+      implicit strategy: SwitchingStrategy[Outer, Inner, Output],
+      allowFlatMap: AllowFlatten
+    ): Output[A] = {
+      strategy.flatten(parent)
+    }
+
+    @inline def flattenSwitch[Output[+_] <: Observable[_]](
+      implicit strategy: SwitchingStrategy[Outer, Inner, Output]
+    ): Output[A] = {
+      strategy.flatten(parent)
+    }
+
+    @inline def flattenMerge[Output[+_] <: Observable[_]](
+      implicit strategy: MergingStrategy[Outer, Inner, Output]
+    ): Output[A] = {
+      strategy.flatten(parent)
+    }
+
+    @inline def flattenCustom[Output[+_] <: Observable[_]](
+      strategy: FlattenStrategy[Outer, Inner, Output]
     ): Output[A] = {
       strategy.flatten(parent)
     }
   }
 
-  implicit val switchStreamStrategy: FlattenStrategy[Observable, EventStream, EventStream] = SwitchStreamStrategy
+  implicit val switchStreamStrategy: SwitchingStrategy[Observable, EventStream, EventStream] = SwitchStreamStrategy
+
+  implicit val switchSignalStreamStrategy: SwitchingStrategy[EventStream, Signal, EventStream] = SwitchSignalStreamStrategy
 
-  implicit val switchSignalStreamStrategy: FlattenStrategy[EventStream, Signal, EventStream] = SwitchSignalStreamStrategy
+  implicit val switchSignalStrategy: SwitchingStrategy[Signal, Signal, Signal] = SwitchSignalStrategy
 
-  implicit val switchSignalStrategy: FlattenStrategy[Signal, Signal, Signal] = SwitchSignalStrategy
+  implicit val mergeStreamsStrategy: MergingStrategy[Observable, EventStream, EventStream] = ConcurrentStreamStrategy
 }
 
 trait ObservableLowPriorityImplicits {
 
-  implicit val switchSignalObservableStrategy: FlattenStrategy[Observable, Signal, Observable] = SwitchSignalObservableStrategy
+  implicit val switchSignalObservableStrategy: SwitchingStrategy[Observable, Signal, Observable] = SwitchSignalObservableStrategy
 }
diff --git a/src/main/scala/com/raquo/airstream/flatten/ConcurrentFutureStream.scala b/src/main/scala/com/raquo/airstream/flatten/ConcurrentFutureStream.scala
deleted file mode 100644
index e69de29b..00000000
diff --git a/src/main/scala/com/raquo/airstream/flatten/FlattenStrategy.scala b/src/main/scala/com/raquo/airstream/flatten/FlattenStrategy.scala
index c9cf2a3a..5d24e1db 100644
--- a/src/main/scala/com/raquo/airstream/flatten/FlattenStrategy.scala
+++ b/src/main/scala/com/raquo/airstream/flatten/FlattenStrategy.scala
@@ -2,43 +2,75 @@ package com.raquo.airstream.flatten
 
 import com.raquo.airstream.core.{EventStream, Observable, Signal}
 
-/** [[Observable.MetaObservable.flatten]] needs an instance of this trait to know how exactly to do the flattening. */
-trait FlattenStrategy[-Outer[+_] <: Observable[_], -Inner[_], +Output[+_] <: Observable[_]] {
+import scala.annotation.implicitNotFound
+
+/** [[Observable.MetaObservable.switchFlatten]] needs an instance of this trait to know how exactly to do the flattening. */
+trait FlattenStrategy[
+  -Outer[+_] <: Observable[_],
+  -Inner[_],
+  +Output[+_] <: Observable[_]
+] {
   /** Must not throw */
   def flatten[A](parent: Outer[Inner[A]]): Output[A]
 }
 
+/** Flatten strategies with semantics of mirroring the latest emitted observable. */
+trait SwitchingStrategy[
+  -Outer[+_] <: Observable[_],
+  -Inner[_],
+  +Output[+_] <: Observable[_]
+] extends FlattenStrategy[Outer, Inner, Output]
+
+/** Flatten strategies with semantics of merging all of the emitted observables. */
+trait MergingStrategy[
+  -Outer[+_] <: Observable[_],
+  -Inner[_],
+  +Output[+_] <: Observable[_]
+] extends FlattenStrategy[Outer, Inner, Output]
+
+@implicitNotFound("\nYou are trying to use Airstream's flatMap operator.\nIt was renamed to flatMapSwitch / flatMapMerge to discourage incorrect usage, especially in for-comprehensions.\nSee https://github.com/raquo/Airstream/#flattening-observables\n\n")
+trait AllowFlatMap
+
+@implicitNotFound("\nYou are trying to use Airstream's flatten operator.\nIt was renamed to flattenSwitch / flattenMerge to clarify intent and discourage incorrect usage, similarly to flatMap.\nSee https://github.com/raquo/Airstream/#flattening-observables\n\n")
+trait AllowFlatten
+
 object FlattenStrategy {
 
+  @deprecated("You are using Airstream's deprecated flatMap operator using FlattenStrategy.flatMapAllowed import. This migration helper will be removed in the next version of Airstream. See https://github.com/raquo/Airstream/#flattening-observables", since = "17.0.0")
+  implicit lazy val allowFlatMap: AllowFlatMap = new AllowFlatMap {}
+
+  @deprecated("You are using Airstream's deprecated flatten operator using FlattenStrategy.flattenAllowed import. This migration helper will be removed in the next version of Airstream. See https://github.com/raquo/Airstream/#flattening-observables", since = "17.0.0")
+  implicit lazy val allowFlatten: AllowFlatten = new AllowFlatten {}
+
   /** See docs for [[SwitchStream]] */
-  object SwitchStreamStrategy extends FlattenStrategy[Observable, EventStream, EventStream] {
+  object SwitchStreamStrategy extends SwitchingStrategy[Observable, EventStream, EventStream] {
     override def flatten[A](parent: Observable[EventStream[A]]): EventStream[A] = {
       new SwitchStream[EventStream[A], A](parent = parent, makeStream = identity)
     }
   }
 
   /** See docs for [[ConcurrentStream]] */
-  object ConcurrentStreamStrategy extends FlattenStrategy[Observable, EventStream, EventStream] {
+  object ConcurrentStreamStrategy extends MergingStrategy[Observable, EventStream, EventStream] {
     override def flatten[A](parent: Observable[EventStream[A]]): EventStream[A] = {
       new ConcurrentStream[A](parent = parent)
     }
   }
 
   /** See docs for [[SwitchSignalStream]] */
-  object SwitchSignalStreamStrategy extends FlattenStrategy[EventStream, Signal, EventStream] {
+  object SwitchSignalStreamStrategy extends SwitchingStrategy[EventStream, Signal, EventStream] {
     override def flatten[A](parent: EventStream[Signal[A]]): EventStream[A] = {
       new SwitchSignalStream(parent)
     }
   }
 
   /** See docs for [[SwitchSignal]] */
-  object SwitchSignalStrategy extends FlattenStrategy[Signal, Signal, Signal] {
+  object SwitchSignalStrategy extends SwitchingStrategy[Signal, Signal, Signal] {
     override def flatten[A](parent: Signal[Signal[A]]): Signal[A] = {
       new SwitchSignal(parent)
     }
   }
 
-  object SwitchSignalObservableStrategy extends FlattenStrategy[Observable, Signal, Observable] {
+  object SwitchSignalObservableStrategy extends SwitchingStrategy[Observable, Signal, Observable] {
     override def flatten[A](parent: Observable[Signal[A]]): Observable[A] = {
       parent.matchStreamOrSignal(
         ifStream = SwitchSignalStreamStrategy.flatten,
diff --git a/src/main/scala/com/raquo/airstream/web/FetchStream.scala b/src/main/scala/com/raquo/airstream/web/FetchStream.scala
index 3995136c..f2465a0c 100644
--- a/src/main/scala/com/raquo/airstream/web/FetchStream.scala
+++ b/src/main/scala/com/raquo/airstream/web/FetchStream.scala
@@ -99,8 +99,8 @@ class FetchBuilder[In, Out](
       maybeAbortStream,
       shouldAbortOnStop,
       emitOnce
-    ).flatMap { promise =>
-      EventStream.fromJsPromise(promise).flatMap(decodeResponse)
+    ).flatMapSwitch { promise =>
+      EventStream.fromJsPromise(promise).flatMapSwitch(decodeResponse)
     }
   }
 }
diff --git a/src/test/scala/com/raquo/airstream/core/GlitchSpec.scala b/src/test/scala/com/raquo/airstream/core/GlitchSpec.scala
index 321fb2db..956d61f3 100644
--- a/src/test/scala/com/raquo/airstream/core/GlitchSpec.scala
+++ b/src/test/scala/com/raquo/airstream/core/GlitchSpec.scala
@@ -426,7 +426,7 @@ class GlitchSpec extends UnitSpec {
     val stateVar = Var(State(Nil))
 
     var n = 0
-    val actions: EventStream[Action] = clickBus.events.flatMap { _ =>
+    val actions: EventStream[Action] = clickBus.events.flatMapSwitch { _ =>
       n += 2
       EventStream.merge(
         EventStream.fromValue(n - 2, emitOnce = true),
diff --git a/src/test/scala/com/raquo/airstream/core/PullResetSignalSpec.scala b/src/test/scala/com/raquo/airstream/core/PullResetSignalSpec.scala
index f72d9165..11575227 100644
--- a/src/test/scala/com/raquo/airstream/core/PullResetSignalSpec.scala
+++ b/src/test/scala/com/raquo/airstream/core/PullResetSignalSpec.scala
@@ -1045,7 +1045,7 @@ class PullResetSignalSpec extends UnitSpec {
         case _ => smallSignal
       }
       .setDisplayName("MetaSignal")
-      .flatten
+      .flattenSwitch
       .setDisplayName("FlatSignal")
       .map(Calculation.log("flat", calculations))
       .setDisplayName("FlatSignal--LOG")
@@ -1136,7 +1136,7 @@ class PullResetSignalSpec extends UnitSpec {
       EventStream.fromSeq("big-1" :: "big-2" :: Nil, emitOnce = true).setDisplayName("BigSeqStream")
     ).setDisplayName("BigMergeStream").startWith("big-0").setDisplayName("BigSignal")
 
-    val flatSignal = outerBus.events.startWith(0).setDisplayName("OuterBus.startWith").flatMap {
+    val flatSignal = outerBus.events.startWith(0).setDisplayName("OuterBus.startWith").flatMapSwitch {
       case i if i >= 10 => bigSignal
       case _ => smallSignal
     }.setDisplayName("FlatSignal").map(Calculation.log("flat", calculations)).setDisplayName("FlatSignal--LOG")
diff --git a/src/test/scala/com/raquo/airstream/errors/EventStreamErrorSpec.scala b/src/test/scala/com/raquo/airstream/errors/EventStreamErrorSpec.scala
index fadd073b..5ff67c60 100644
--- a/src/test/scala/com/raquo/airstream/errors/EventStreamErrorSpec.scala
+++ b/src/test/scala/com/raquo/airstream/errors/EventStreamErrorSpec.scala
@@ -347,7 +347,7 @@ class EventStreamErrorSpec extends UnitSpec with BeforeAndAfter {
 
     val bus  = new EventBus[Int]
 
-    val stream = bus.events.flatMap(EventStream.fromValue(_, emitOnce = true))
+    val stream = bus.events.flatMapSwitch(EventStream.fromValue(_, emitOnce = true))
 
     val effects = mutable.Buffer[Effect[_]]()
 
@@ -387,7 +387,7 @@ class EventStreamErrorSpec extends UnitSpec with BeforeAndAfter {
 
     val myVar  = Var.fromTry[Int](Failure(err))
 
-    val stream = myVar.signal.flatMap(EventStream.fromValue(_, emitOnce = true))
+    val stream = myVar.signal.flatMapSwitch(EventStream.fromValue(_, emitOnce = true))
 
     val effects = mutable.Buffer[Effect[_]]()
 
diff --git a/src/test/scala/com/raquo/airstream/errors/SignalErrorSpec.scala b/src/test/scala/com/raquo/airstream/errors/SignalErrorSpec.scala
index 5346d552..c54453c6 100644
--- a/src/test/scala/com/raquo/airstream/errors/SignalErrorSpec.scala
+++ b/src/test/scala/com/raquo/airstream/errors/SignalErrorSpec.scala
@@ -354,7 +354,7 @@ class SignalErrorSpec extends UnitSpec with BeforeAndAfter {
 
     val myVar = Var(0)
 
-    val stream = myVar.signal.flatMap(Val(_))
+    val stream = myVar.signal.flatMapSwitch(Val(_))
 
     val effects = mutable.Buffer[Effect[_]]()
 
@@ -400,7 +400,7 @@ class SignalErrorSpec extends UnitSpec with BeforeAndAfter {
     val myVar = Var.fromTry[Int](Failure(err))
 
     // @TODO[Airstream] Add Signal.fromValue / fromTry that creates a Val
-    val stream = myVar.signal.flatMap(Val(_))
+    val stream = myVar.signal.flatMapSwitch(Val(_))
 
     val effects = mutable.Buffer[Effect[_]]()
 
diff --git a/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenFutureSpec.scala b/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenFutureSpec.scala
index 19b395d5..022996dc 100644
--- a/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenFutureSpec.scala
+++ b/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenFutureSpec.scala
@@ -37,7 +37,7 @@ class EventStreamFlattenFutureSpec extends AsyncUnitSpec {
     val promise5 = makePromise()
 
     val futureBus = new EventBus[Future[Int]]()
-    val stream = futureBus.events.flatMap(EventStream.fromFuture(_))
+    val stream = futureBus.events.flatMapSwitch(EventStream.fromFuture(_))
 
     stream.addObserver(obs)
 
diff --git a/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenSpec.scala b/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenSpec.scala
index 84ab05f5..f914f3ab 100644
--- a/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenSpec.scala
+++ b/src/test/scala/com/raquo/airstream/flatten/EventStreamFlattenSpec.scala
@@ -1,16 +1,16 @@
 package com.raquo.airstream.flatten
 
-import com.raquo.airstream.AsyncUnitSpec
+import com.raquo.airstream.{AsyncUnitSpec, Matchers}
 import com.raquo.airstream.core.{EventStream, Observer}
 import com.raquo.airstream.eventbus.EventBus
 import com.raquo.airstream.fixtures.{Calculation, Effect, TestableOwner}
-import com.raquo.airstream.flatten.FlattenStrategy.ConcurrentStreamStrategy
 import com.raquo.airstream.ownership.Owner
 import com.raquo.airstream.state.Var
 
+import scala.annotation.nowarn
 import scala.collection.mutable
 
-class EventStreamFlattenSpec extends AsyncUnitSpec {
+class EventStreamFlattenSpec extends AsyncUnitSpec with Matchers {
 
   private val done = assert(true)
 
@@ -25,7 +25,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
         .map { v =>
           EventStream.fromSeq(Seq(v * 3), emitOnce = true).setDisplayName(s"INT-FS-$v")
         }.setDisplayName("META")
-        .flatten.setDisplayName("FLAT")
+        .flattenSwitch.setDisplayName("FLAT")
 
     val effects = mutable.Buffer[Effect[_]]()
     val obs0 = Observer[Int](newValue => effects += Effect("obs0", newValue)).setDisplayName("obs0")
@@ -47,7 +47,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
           EventStream.fromValue(v * 3, emitOnce = true).setDisplayName(s"S-${v}")
         }
         .setDisplayName("MO")
-        .flatten.setDisplayName("FS")
+        .flattenSwitch.setDisplayName("FS")
 
     val effects = mutable.Buffer[Effect[_]]()
     val obs = Observer[Int](v => effects += Effect("obs0", v)).setDisplayName("obs")
@@ -77,9 +77,9 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
         .map { v =>
           EventStream.fromSeq(Seq(v * 3), emitOnce = true).map { vv =>
             EventStream.fromSeq(Seq(vv * 7), emitOnce = true)
-          }.flatten
+          }.flattenSwitch
         }
-        .flatten
+        .flattenSwitch
 
     val effects = mutable.Buffer[Effect[_]]()
     val subscription0 = flatStream.foreach(newValue => effects += Effect("obs0", newValue))
@@ -114,7 +114,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
         .map { v =>
           delayedStream(range2, interval = 6, _ * v)
         }
-        .flatten
+        .flattenSwitch
 
     val effects = mutable.Buffer[Effect[_]]()
     val subscription0 = flatStream.foreach(newValue => effects += Effect("obs0", newValue))
@@ -146,9 +146,9 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
         .map { v =>
           delayedStream(range2, interval = 6, _ * v).map { vv =>
             EventStream.fromFuture(delay(1)(vv * 7))
-          }.flatten
+          }.flattenSwitch
         }
-        .flatten
+        .flattenSwitch
 
     val effects = mutable.Buffer[Effect[_]]()
     val subscription0 = flatStream.foreach(newValue => effects += Effect("obs0", newValue))
@@ -161,7 +161,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     }
   }
 
-  it("sync flatMap") {
+  it("sync flatMapSwitch") {
 
     implicit val owner: Owner = new TestableOwner
 
@@ -169,7 +169,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     val stream = EventStream.fromSeq(range, emitOnce = true)
     val flatStream =
       stream
-        .flatMap { v =>
+        .flatMapSwitch { v =>
           EventStream.fromSeq(Seq(v * 3), emitOnce = true)
         }
 
@@ -180,7 +180,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     effects.toList shouldBe range.map(i => Effect("obs0", i * 3))
   }
 
-  it("sync three-level flatMap") {
+  it("sync three-level flatMapSwitch") {
 
     implicit val owner: Owner = new TestableOwner
 
@@ -188,8 +188,8 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     val stream = EventStream.fromSeq(range, emitOnce = true)
     val flatStream =
       stream
-        .flatMap { v =>
-          EventStream.fromSeq(Seq(v * 3), emitOnce = true).flatMap { vv =>
+        .flatMapSwitch { v =>
+          EventStream.fromSeq(Seq(v * 3), emitOnce = true).flatMapSwitch { vv =>
             EventStream.fromSeq(Seq(vv * 7), emitOnce = true)
           }
         }
@@ -205,7 +205,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     * emitted by the inner delayedStream are processed. Just because the interval is set to 6ms
     * does not mean that this is what it will be. It's merely the lower bound.
     */
-  it("from-future flatMap") {
+  it("from-future flatMapSwitch") {
     implicit val owner: Owner = new TestableOwner
 
     val range1 = 1 to 3
@@ -214,7 +214,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
 
     val flatStream =
       stream
-        .flatMap { v =>
+        .flatMapSwitch { v =>
           delayedStream(range2, interval = 6, _ * v)
         }
 
@@ -236,7 +236,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     * emitted by the inner delayedStream are processed. Just because the interval is set to 6ms
     * does not mean that this is what it will be. It's merely the lower bound.
     */
-  it("three-level from-future flatMap") {
+  it("three-level from-future flatMapSwitch") {
     implicit val owner: Owner = new TestableOwner
 
     val range1 = 1 to 3
@@ -245,8 +245,8 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
 
     val flatStream =
       stream
-        .flatMap { v =>
-          delayedStream(range2, interval = 6, _ * v).flatMap { vv =>
+        .flatMapSwitch { v =>
+          delayedStream(range2, interval = 6, _ * v).flatMapSwitch { vv =>
             EventStream.fromFuture(delay(1)(vv * 7))
           }
         }
@@ -277,7 +277,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
 
     val mergeBus = new EventBus[EventStream[Int]]
 
-    val mergeStream = mergeBus.events.flatten(ConcurrentStreamStrategy).map(Calculation.log("merge", calculations))
+    val mergeStream = mergeBus.events.flattenMerge.map(Calculation.log("merge", calculations))
 
     val sub1 = mergeStream.addObserver(Observer.empty)
 
@@ -396,7 +396,7 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     val mergeSignal = streamVar
       .signal
       .distinct
-      .flatten(ConcurrentStreamStrategy)
+      .flattenMerge
       .map(Calculation.log("merge", calculations))
 
     val sub1 = mergeSignal.addObserver(Observer.empty)
@@ -501,4 +501,26 @@ class EventStreamFlattenSpec extends AsyncUnitSpec {
     done
   }
 
+  it("legacy flatMap and flatten methods") {
+
+    val bus = new EventBus[Int]
+
+    assertTypeError("bus.events.flatMap(_ => EventStream.fromValue(1))")
+
+    @nowarn("cat=deprecation")
+    def flatMapCompileCheck() = {
+      import com.raquo.airstream.flatten.FlattenStrategy.allowFlatMap
+      bus.events.flatMap(_ => EventStream.fromValue(1))
+    }
+
+    assertTypeError("bus.events.map(_ => EventStream.fromValue(1)).flatten")
+
+    @nowarn("cat=deprecation")
+    def flattenCompileCheck() = {
+      import com.raquo.airstream.flatten.FlattenStrategy.allowFlatten
+      bus.events.map(_ => EventStream.fromValue(1)).flatten
+    }
+
+    done
+  }
 }
diff --git a/src/test/scala/com/raquo/airstream/flatten/SignalFlattenFutureSpec.scala b/src/test/scala/com/raquo/airstream/flatten/SignalFlattenFutureSpec.scala
index 380e04ca..96570a83 100644
--- a/src/test/scala/com/raquo/airstream/flatten/SignalFlattenFutureSpec.scala
+++ b/src/test/scala/com/raquo/airstream/flatten/SignalFlattenFutureSpec.scala
@@ -36,7 +36,7 @@ class SignalFlattenFutureSpec extends AsyncUnitSpec {
       val futureBus = new EventBus[Future[Int]]()
       val signal = futureBus.events
         .startWith(promise0.future)
-        .flatMap(Signal.fromFuture(_, initial = -200))
+        .flatMapSwitch(Signal.fromFuture(_, initial = -200))
 
       signal.addObserver(obs)
 
@@ -93,7 +93,7 @@ class SignalFlattenFutureSpec extends AsyncUnitSpec {
 
       val futureBus = new EventBus[Future[Int]]()
 
-      val signal = futureBus.events.startWith(promise0.future).flatMap(Signal.fromFuture(_, initial = -200))
+      val signal = futureBus.events.startWith(promise0.future).flatMapSwitch(Signal.fromFuture(_, initial = -200))
       promise0.success(-100)
 
       signal.addObserver(obs)
@@ -148,7 +148,7 @@ class SignalFlattenFutureSpec extends AsyncUnitSpec {
       promise0.success(-100)
 
       delay {
-        val signal = futureBus.events.startWith(promise0.future).flatMap(Signal.fromFuture(_, initial = -200))
+        val signal = futureBus.events.startWith(promise0.future).flatMapSwitch(Signal.fromFuture(_, initial = -200))
         signal.addObserver(obs)
 
         effects shouldBe mutable.Buffer(Effect("obs", -200))
diff --git a/src/test/scala/com/raquo/airstream/flatten/SwitchSignalSpec.scala b/src/test/scala/com/raquo/airstream/flatten/SwitchSignalSpec.scala
index 4088706f..19a78280 100644
--- a/src/test/scala/com/raquo/airstream/flatten/SwitchSignalSpec.scala
+++ b/src/test/scala/com/raquo/airstream/flatten/SwitchSignalSpec.scala
@@ -26,7 +26,7 @@ class SwitchSignalSpec extends UnitSpec {
 
     val metaVar = Var(sourceSignals(0))
 
-    val $latestNumber = metaVar.signal.flatten // SwitchSignalStrategy is the default (provided implicitly)
+    val $latestNumber = metaVar.signal.flattenSwitch // SwitchSignalStrategy is the default (provided implicitly)
 
     val flattenObserver = Observer[Int](effects += Effect("flattened-obs", _))
 
@@ -228,7 +228,7 @@ class SwitchSignalSpec extends UnitSpec {
     val flatSignal = outerBus.events.setDisplayName("outerBus.events").startWith(0).setDisplayName("outerBus.signal").map {
       case i if i >= 10 => bigSignal
       case _ => smallSignal
-    }.setDisplayName("outerBus.meta").flatten.setDisplayName("flatSignal").map(Calculation.log("flat", calculations))
+    }.setDisplayName("outerBus.meta").flattenSwitch.setDisplayName("flatSignal").map(Calculation.log("flat", calculations))
 
     // --
 
@@ -374,7 +374,7 @@ class SwitchSignalSpec extends UnitSpec {
 
     val brokenSignal =
       intSignal
-        .flatMap { num =>
+        .flatMapSwitch { num =>
           if (num < 1000) {
             smallI += 1
             intSignal.map("small: " + _).setDisplayName(s"small-$smallI") //.debugLogLifecycle()
diff --git a/src/test/scala/com/raquo/airstream/flatten/SwitchSignalStreamSpec.scala b/src/test/scala/com/raquo/airstream/flatten/SwitchSignalStreamSpec.scala
index 2d531716..f1bb3558 100644
--- a/src/test/scala/com/raquo/airstream/flatten/SwitchSignalStreamSpec.scala
+++ b/src/test/scala/com/raquo/airstream/flatten/SwitchSignalStreamSpec.scala
@@ -27,7 +27,7 @@ class SwitchSignalStreamSpec extends UnitSpec {
 
     val metaBus = new EventBus[Signal[Int]]
 
-    val $latestNumber = metaBus.events.flatten // SwitchSignalStreamStrategy is the default (provided implicitly)
+    val $latestNumber = metaBus.events.flattenSwitch // SwitchSignalStreamStrategy is the default (provided implicitly)
 
     val flattenObserver = Observer[Int](effects += Effect("flattened-obs", _))
 
@@ -213,7 +213,7 @@ class SwitchSignalStreamSpec extends UnitSpec {
 
     val metaBus = new EventBus[Signal[Int]]
 
-    val $latestNumber = metaBus.events.flatten // SwitchSignalStreamStrategy is the default (provided implicitly)
+    val $latestNumber = metaBus.events.flattenSwitch // SwitchSignalStreamStrategy is the default (provided implicitly)
 
     val flattenObserver = Observer[Int](effects += Effect("flattened-obs", _))
 
diff --git a/src/test/scala/com/raquo/airstream/flatten/SwitchStreamSpec.scala b/src/test/scala/com/raquo/airstream/flatten/SwitchStreamSpec.scala
index 44318b00..857cd166 100644
--- a/src/test/scala/com/raquo/airstream/flatten/SwitchStreamSpec.scala
+++ b/src/test/scala/com/raquo/airstream/flatten/SwitchStreamSpec.scala
@@ -27,7 +27,7 @@ class SwitchStreamSpec extends UnitSpec {
       case (bus, index) => bus.events.map(Calculation.log(s"source-$index", calculations))
     }
 
-    val $latestNumber = metaBus.events.flatten // SwitchStreamStrategy is the default (provided implicitly)
+    val $latestNumber = metaBus.events.flattenSwitch // SwitchStreamStrategy is the default (provided implicitly)
 
     val flattenObserver = Observer[Int](effects += Effect("flattened-obs", _))
 
@@ -156,7 +156,7 @@ class SwitchStreamSpec extends UnitSpec {
 
     val metaVar = Var[EventStream[Int]](sourceStreams(0))
 
-    val $latestNumber = metaVar.signal.flatten(SwitchStreamStrategy)
+    val $latestNumber = metaVar.signal.flattenSwitch(SwitchStreamStrategy)
 
     val flattenObserver = Observer[Int](effects += Effect("flattened-obs", _))
 
@@ -313,7 +313,7 @@ class SwitchStreamSpec extends UnitSpec {
       EventStream.fromSeq("big-1" :: "big-2" :: Nil, emitOnce = true)
     )
 
-    val flatStream = outerBus.events.flatMap {
+    val flatStream = outerBus.events.flatMapSwitch {
       case i if i >= 10 => bigStream
       case _ => smallStream
     }.map(Calculation.log("flat", calculations))
@@ -457,7 +457,7 @@ class SwitchStreamSpec extends UnitSpec {
       case i if i >= 10 => bigStream
       case _ => smallStream
     }.setDisplayName("outer-meta")
-      .flatten.setDisplayName("outer-flat")
+      .flattenSwitch.setDisplayName("outer-flat")
       .map(Calculation.log("flat", calculations)).setDisplayName("outer-flat-map")
 
     // --
@@ -598,7 +598,7 @@ class SwitchStreamSpec extends UnitSpec {
 
     val brokenSignal =
       intStream
-        .flatMap { num =>
+        .flatMapSwitch { num =>
           if (num < 1000) {
             smallI += 1
             intStream.map("small: " + _).setDisplayName(s"small-$smallI") //.debugLogLifecycle()
diff --git a/src/test/scala/com/raquo/airstream/syntax/SyntaxSpec.scala b/src/test/scala/com/raquo/airstream/syntax/SyntaxSpec.scala
index 1875edd6..bc4853f0 100644
--- a/src/test/scala/com/raquo/airstream/syntax/SyntaxSpec.scala
+++ b/src/test/scala/com/raquo/airstream/syntax/SyntaxSpec.scala
@@ -78,12 +78,12 @@ class SyntaxSpec extends UnitSpec {
     val bus = new EventBus[Int]
 
     locally {
-      val flatStream = bus.events.flatMap(a => EventStream.fromFuture(Future.successful(a)))
+      val flatStream = bus.events.flatMapSwitch(a => EventStream.fromFuture(Future.successful(a)))
       flatStream: EventStream[Int]
     }
 
     locally {
-      val flatSignal = bus.events.startWith(0).flatMap(a => Signal.fromFuture(Future.successful(a), initial = 0))
+      val flatSignal = bus.events.startWith(0).flatMapSwitch(a => Signal.fromFuture(Future.successful(a), initial = 0))
       flatSignal: Signal[Int]
     }
   }