diff --git a/main/src/main/scala/sbt/Project.scala b/main/src/main/scala/sbt/Project.scala
index ea3a6b942..bac52a877 100755
--- a/main/src/main/scala/sbt/Project.scala
+++ b/main/src/main/scala/sbt/Project.scala
@@ -121,7 +121,45 @@ sealed trait ProjectDefinition[PR <: ProjectReference] {
     if (ts.isEmpty) Nil else s"$label: $ts" :: Nil
 }
 
-sealed trait Project extends ProjectDefinition[ProjectReference] {
+trait CompositeProject {
+  def componentProjects: Seq[Project]
+}
+
+private[sbt] object CompositeProject {
+
+  /**
+   *  Expand user defined projects with the component projects of `compositeProjects`.
+   *
+   *  If two projects with the same id appear in the user defined projects and
+   *  in `compositeProjects.componentProjects`, the user defined project wins.
+   *  This is necessary for backward compatibility with the idioms:
+   *  {{{
+   *    lazy val foo = crossProject
+   *    lazy val fooJS = foo.js.settings(...)
+   *    lazy val fooJVM = foo.jvm.settings(...)
+   *  }}}
+   *  and the rarer:
+   *  {{{
+   *    lazy val fooJS = foo.js.settings(...)
+   *    lazy val foo = crossProject
+   *    lazy val fooJVM = foo.jvm.settings(...)
+   *  }}}
+   */
+  def expand(compositeProjects: Seq[CompositeProject]): Seq[Project] = {
+    val userProjects = compositeProjects.collect { case p: Project => p }
+    for (p <- compositeProjects.flatMap(_.componentProjects)) yield {
+      userProjects.find(_.id == p.id) match {
+        case Some(userProject) => userProject
+        case None              => p
+      }
+    }
+  }.distinct
+
+}
+
+sealed trait Project extends ProjectDefinition[ProjectReference] with CompositeProject {
+  def componentProjects: Seq[Project] = this :: Nil
+
   private[sbt] def copy(
       id: String = id,
       base: File = base,
diff --git a/main/src/main/scala/sbt/internal/BuildDef.scala b/main/src/main/scala/sbt/internal/BuildDef.scala
index dd1d11aec..d4a435ab5 100644
--- a/main/src/main/scala/sbt/internal/BuildDef.scala
+++ b/main/src/main/scala/sbt/internal/BuildDef.scala
@@ -17,7 +17,8 @@ import sbt.internal.inc.ReflectUtilities
 
 trait BuildDef {
   def projectDefinitions(@deprecated("unused", "") baseDirectory: File): Seq[Project] = projects
-  def projects: Seq[Project] = ReflectUtilities.allVals[Project](this).values.toSeq
+  def projects: Seq[Project] =
+    CompositeProject.expand(ReflectUtilities.allVals[CompositeProject](this).values.toSeq)
   // TODO: Should we grab the build core settings here or in a plugin?
   def settings: Seq[Setting[_]] = Defaults.buildCore
   def buildLoaders: Seq[BuildLoader.Components] = Nil
diff --git a/main/src/main/scala/sbt/internal/EvaluateConfigurations.scala b/main/src/main/scala/sbt/internal/EvaluateConfigurations.scala
index 85b40293c..26e5348ff 100644
--- a/main/src/main/scala/sbt/internal/EvaluateConfigurations.scala
+++ b/main/src/main/scala/sbt/internal/EvaluateConfigurations.scala
@@ -151,10 +151,12 @@ private[sbt] object EvaluateConfigurations {
     val allGeneratedFiles = (definitions.generated ++ dslEntries.flatMap(_.generated))
     loader =>
       {
-        val projects =
-          definitions.values(loader).collect {
-            case p: Project => resolveBase(file.getParentFile, p)
+        val projects = {
+          val compositeProjects = definitions.values(loader).collect {
+            case p: CompositeProject => p
           }
+          CompositeProject.expand(compositeProjects).map(resolveBase(file.getParentFile, _))
+        }
         val (settingsRaw, manipulationsRaw) =
           dslEntries map (_.result apply loader) partition {
             case DslEntry.ProjectSettings(_) => true
@@ -280,7 +282,10 @@ private[sbt] object EvaluateConfigurations {
   }
 
   private[this] def extractedValTypes: Seq[String] =
-    Seq(classOf[Project], classOf[InputKey[_]], classOf[TaskKey[_]], classOf[SettingKey[_]])
+    Seq(classOf[CompositeProject],
+        classOf[InputKey[_]],
+        classOf[TaskKey[_]],
+        classOf[SettingKey[_]])
       .map(_.getName)
 
   private[this] def evaluateDefinitions(eval: Eval,
diff --git a/notes/1.2.0/introduce-CompositeProject.md b/notes/1.2.0/introduce-CompositeProject.md
new file mode 100644
index 000000000..759d75a71
--- /dev/null
+++ b/notes/1.2.0/introduce-CompositeProject.md
@@ -0,0 +1,9 @@
+
+[#3042]: https://github.com/sbt/sbt/issues/3042
+[#4056]: https://github.com/sbt/sbt/pull/4056
+
+### Introduce CompositeProject
+
+### Improvements
+
+- Support for: `lazy val foo = someCompositeProject` (e.g.`CrossProject`)  [#3042][]/[#4056][]
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/build.sbt b/sbt/src/sbt-test/project/sbt-composite-projects/build.sbt
new file mode 100644
index 000000000..1537b2610
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/build.sbt
@@ -0,0 +1,36 @@
+import sbt.internal.AddSettings
+import sbt.CompositeProject
+
+lazy val check = taskKey[Unit]("check")
+
+// Based on sbt-file-projects test
+lazy val foo = new CompositeProject
+{
+  val jvm = Project.apply("jvm", new File("jvm")).settings(version := "0.1.0") // this one needs to win
+  val js = Project.apply("js", new File("js")).settings(version := "0.1.0") // this one needs to win
+  def componentProjects: Seq[Project] = Seq(jvm, js)
+}
+
+lazy val fooJS = foo.js
+lazy val fooJVM = foo.jvm
+
+lazy val bar = project
+  .dependsOn(foo.jvm)
+
+val g = taskKey[Unit]("A task in the root project")
+g := println("Hello.")
+
+
+check := {
+  val verJvm = (version in foo.jvm).?.value
+  assert (verJvm == Some("0.1.0"))
+
+  val verFooJvm = (version in fooJVM).?.value
+  assert (verFooJvm == Some("0.1.0"))
+
+  val verJs = (version in foo.js).?.value
+  assert (verJs == Some("0.1.0"))
+
+  val verFooJs = (version in fooJS).?.value
+  assert (verFooJs == Some("0.1.0"))
+}
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/changes/basic.sbt b/sbt/src/sbt-test/project/sbt-composite-projects/changes/basic.sbt
new file mode 100644
index 000000000..c128b140e
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/changes/basic.sbt
@@ -0,0 +1 @@
+lazy val root = (project in file("."))
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/changes/shadow.sbt b/sbt/src/sbt-test/project/sbt-composite-projects/changes/shadow.sbt
new file mode 100644
index 000000000..0713e7f4b
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/changes/shadow.sbt
@@ -0,0 +1,36 @@
+import sbt.internal.AddSettings
+import sbt.CompositeProject
+
+lazy val check = taskKey[Unit]("check")
+
+// Based on sbt-file-projects test
+lazy val foo = new CompositeProject
+{
+  val jvm = Project.apply("jvm", new File("jvm")).settings(version := "0.1.0")
+  val js = Project.apply("js", new File("js")).settings(version := "0.1.0") // this one needs to win
+  def componentProjects: Seq[Project] = Seq(jvm, js)
+}
+
+lazy val fooJS = foo.js
+lazy val fooJVM = foo.jvm.settings(version := "0.2.0") // this one needs to win
+
+lazy val bar = project
+  .dependsOn(foo.jvm)
+
+val g = taskKey[Unit]("A task in the root project")
+g := println("Hello.")
+
+
+check := {
+  val verJvm = (version in foo.jvm).?.value
+  assert (verJvm == Some("0.2.0"))
+
+  val verFooJvm = (version in fooJVM).?.value
+  assert (verFooJvm == Some("0.2.0"))
+
+  val verJs = (version in foo.js).?.value
+  assert (verJs == Some("0.1.0"))
+
+  val verFooJs = (version in fooJS).?.value
+  assert (verFooJs == Some("0.1.0"))
+}
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/changes/shadowLazy.sbt b/sbt/src/sbt-test/project/sbt-composite-projects/changes/shadowLazy.sbt
new file mode 100644
index 000000000..492fbd621
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/changes/shadowLazy.sbt
@@ -0,0 +1,37 @@
+import sbt.internal.AddSettings
+import sbt.CompositeProject
+
+lazy val check = taskKey[Unit]("check")
+
+lazy val fooJS = foo.js.settings(version := "0.2.1") // this one needs to win
+
+// Based on sbt-file-projects test
+lazy val foo = new CompositeProject
+{
+  val jvm = Project.apply("jvm", new File("jvm")).settings(version := "0.1.0")
+  val js = Project.apply("js", new File("js")).settings(version := "0.1.0")
+  def componentProjects: Seq[Project] = Seq(jvm, js)
+}
+
+lazy val fooJVM = foo.jvm.settings(version := "0.2.0") // this one needs to win
+
+lazy val bar = project
+  .dependsOn(foo.jvm)
+
+val g = taskKey[Unit]("A task in the root project")
+g := println("Hello.")
+
+
+check := {
+  val verJvm = (version in foo.jvm).?.value
+  assert (verJvm == Some("0.2.0"))
+
+  val verFooJvm = (version in fooJVM).?.value
+  assert (verFooJvm == Some("0.2.0"))
+
+  val verJs = (version in foo.js).?.value
+  assert (verJs == Some("0.2.1"))
+
+  val verFooJs = (version in fooJS).?.value
+  assert (verFooJs == Some("0.2.1"))
+}
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/js/build.sbt b/sbt/src/sbt-test/project/sbt-composite-projects/js/build.sbt
new file mode 100644
index 000000000..0ddd49e30
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/js/build.sbt
@@ -0,0 +1,2 @@
+val h = taskKey[Unit]("A task in project 'js'")
+h := println("Hello.")
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/jvm/A.scala b/sbt/src/sbt-test/project/sbt-composite-projects/jvm/A.scala
new file mode 100644
index 000000000..528ffce71
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/jvm/A.scala
@@ -0,0 +1 @@
+object A
\ No newline at end of file
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/jvm/a.sbt b/sbt/src/sbt-test/project/sbt-composite-projects/jvm/a.sbt
new file mode 100644
index 000000000..9b73a55bf
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/jvm/a.sbt
@@ -0,0 +1,2 @@
+val aa = taskKey[Unit]("A task in the 'jvm' project")
+aa := println("Hello.")
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/other.sbt b/sbt/src/sbt-test/project/sbt-composite-projects/other.sbt
new file mode 100644
index 000000000..94e5f2363
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/other.sbt
@@ -0,0 +1 @@
+val c = project
diff --git a/sbt/src/sbt-test/project/sbt-composite-projects/test b/sbt/src/sbt-test/project/sbt-composite-projects/test
new file mode 100644
index 000000000..34bc8097d
--- /dev/null
+++ b/sbt/src/sbt-test/project/sbt-composite-projects/test
@@ -0,0 +1,40 @@
+> g
+-> root/compile
+> jvm/compile
+> jvm/aa
+> js/compile
+> js/h
+> c/compile
+> bar/compile
+
+$ copy-file changes/basic.sbt basic.sbt
+> reload
+> g
+> root/compile
+> jvm/compile
+> jvm/aa
+> js/compile
+> js/h
+> c/compile
+> bar/compile
+> check
+
+$ copy-file changes/shadow.sbt build.sbt
+> reload
+> jvm/compile
+> jvm/aa
+> js/compile
+> js/h
+> c/compile
+> bar/compile
+> check
+
+$ copy-file changes/shadowLazy.sbt build.sbt
+> reload
+> jvm/compile
+> jvm/aa
+> js/compile
+> js/h
+> c/compile
+> bar/compile
+> check
\ No newline at end of file