diff --git a/compile/ComponentCompiler.scala b/compile/ComponentCompiler.scala
index 980042158..5af5e0db4 100644
--- a/compile/ComponentCompiler.scala
+++ b/compile/ComponentCompiler.scala
@@ -20,21 +20,18 @@ class ComponentCompiler(compiler: RawCompiler, manager: ComponentManager)
 	def apply(id: String): File =
 	{
 		val binID = binaryID(id)
-		try { manager.file(binID) }
-		catch { case e: InvalidComponent => compileAndInstall(id, binID) }
+		manager.file(binID)( new IfMissing.Define(true, compileAndInstall(id, binID)) )
 	}
 	def clearCache(id: String): Unit = manager.clearCache(binaryID(id))
 	protected def binaryID(id: String) = id + binSeparator + compiler.scalaInstance.actualVersion
-	protected def compileAndInstall(id: String, binID: String): File =
+	protected def compileAndInstall(id: String, binID: String)
 	{
 		val srcID = id + srcExtension
 		withTemporaryDirectory { binaryDirectory =>
 			val targetJar = new File(binaryDirectory, id + ".jar")
-			compileSources(manager.files(srcID), targetJar, id)
+			compileSources(manager.files(srcID)(IfMissing.Fail), targetJar, id)
 			manager.define(binID, Seq(targetJar))
-			manager.cache(binID)
 		}
-		manager.file(binID)
 	}
 	/** Extract sources from source jars, compile them with the xsbti interfaces on the classpath, and package the compiled classes and
 	* any resources from the source jars into a final jar.*/
@@ -45,7 +42,7 @@ class ComponentCompiler(compiler: RawCompiler, manager: ComponentManager)
 			val extractedSources = (Set[File]() /: sourceJars) { (extracted, sourceJar)=> extracted ++ unzip(sourceJar, dir) }
 			val (sourceFiles, resources) = extractedSources.partition(_.getName.endsWith(".scala"))
 			withTemporaryDirectory { outputDirectory =>
-				val xsbtiJars = manager.files(xsbtiID)
+				val xsbtiJars = manager.files(xsbtiID)(IfMissing.Fail)
 				manager.log.info("'" + id + "' not yet compiled for Scala " + compiler.scalaInstance.actualVersion + ". Compiling...")
 				try { compiler(Set() ++ sourceFiles, Set() ++ xsbtiJars, outputDirectory, Nil, true) }
 				catch { case e: xsbti.CompileFailed => throw new CompileFailed(e.arguments, "Error compiling sbt component '" + id + "'") }
diff --git a/ivy/ComponentManager.scala b/ivy/ComponentManager.scala
index e420ffe22..483a0bb31 100644
--- a/ivy/ComponentManager.scala
+++ b/ivy/ComponentManager.scala
@@ -1,6 +1,7 @@
 package xsbt
 
-import java.io.File
+import java.io.{File,FileOutputStream}
+import ComponentManager.lock
 
 /** A component manager provides access to the pieces of xsbt that are distributed as components.
 * There are two types of components.  The first type is compiled subproject jars with their dependencies.
@@ -14,33 +15,73 @@ import java.io.File
 class ComponentManager(provider: xsbti.ComponentProvider, val log: IvyLogger) extends NotNull
 {
 	/** Get all of the files for component 'id', throwing an exception if no files exist for the component. */
-	def files(id: String): Iterable[File] =
+	def files(id: String)(ifMissing: IfMissing): Iterable[File] =
 	{
-		val existing = provider.component(id)
-		val fs = if(existing.isEmpty) { update(id); provider.component(id) } else existing
-		if(!fs.isEmpty) fs else invalid("Could not find required component '" + id + "'")
+		def fromGlobal =
+			lockGlobalCache {
+				try { update(id); getOrElse(createAndCache) }
+				catch { case e: NotInCache => createAndCache }
+			}
+		def getOrElse(orElse: => Iterable[File]) =
+		{
+			val existing = provider.component(id)
+			if(existing.isEmpty) orElse else existing
+		}
+		def notFound = invalid("Could not find required component '" + id + "'")
+		def createAndCache =
+			ifMissing match {
+				case IfMissing.Fail => notFound
+				case d: IfMissing.Define =>
+					d()
+					if(d.cache) cache(id)
+					getOrElse(notFound)
+			}
+		
+		lockLocalCache { getOrElse(fromGlobal) }
 	}
+	private def lockLocalCache[T](action: => T): T = lock("local cache", provider.lockFile, log) ( action )
+	private def lockGlobalCache[T](action: => T): T = lock("global cache", IvyCache.lockFile, log)( action )
 	/** Get the file for component 'id', throwing an exception if no files or multiple files exist for the component. */
-	def file(id: String): File =
-		files(id).toList match {
+	def file(id: String)(ifMissing: IfMissing): File =
+		files(id)(ifMissing).toList match {
 			case x :: Nil => x
 			case xs => invalid("Expected single file for component '" + id + "', found: " + xs.mkString(", "))
 		}
 	private def invalid(msg: String) = throw new InvalidComponent(msg)
 	private def invalid(e: NotInCache) = throw new InvalidComponent(e.getMessage, e)
 
-	def define(id: String, files: Iterable[File]) = provider.defineComponent(id, files.toSeq.toArray)
+	def define(id: String, files: Iterable[File]) = lockLocalCache { provider.defineComponent(id, files.toSeq.toArray) }
 	/** Retrieve the file for component 'id' from the local repository. */
-	def update(id: String): Unit =
-		try { IvyCache.withCachedJar(sbtModuleID(id), log)(jar => define(id, Seq(jar)) ) }
-		catch { case e: NotInCache => invalid(e) }
+	private def update(id: String): Unit = IvyCache.withCachedJar(sbtModuleID(id), log)(jar => define(id, Seq(jar)) )
 
-	def sbtModuleID(id: String) = ModuleID("org.scala-tools.sbt", id, xsbti.Versions.Sbt)
+	private def sbtModuleID(id: String) = ModuleID("org.scala-tools.sbt", id, xsbti.Versions.Sbt)
 	/** Install the files for component 'id' to the local repository.  This is usually used after writing files to the directory returned by 'location'. */
-	def cache(id: String): Unit = IvyCache.cacheJar(sbtModuleID(id), file(id), log)
-	def clearCache(id: String): Unit = IvyCache.clearCachedJar(sbtModuleID(id), log)
+	def cache(id: String): Unit = IvyCache.cacheJar(sbtModuleID(id), file(id)(IfMissing.Fail), log)
+	def clearCache(id: String): Unit = lockGlobalCache { IvyCache.clearCachedJar(sbtModuleID(id), log) }
 }
 class InvalidComponent(msg: String, cause: Throwable) extends RuntimeException(msg, cause)
 {
 	def this(msg: String) = this(msg, null)
+}
+sealed trait IfMissing extends NotNull
+object IfMissing
+{
+	object Fail extends IfMissing
+	final class Define(val cache: Boolean, define: => Unit) extends IfMissing { def apply() = define }
+}
+object ComponentManager
+{
+	def lock[T](label: String, file: File, log: IvyLogger)(action: => T): T =
+	{
+		synchronized {
+			val channel = new FileOutputStream(file).getChannel
+			try {
+				val freeLock = channel.tryLock
+				val lock = if(freeLock eq null) { log.info("Waiting for " + label + " to be available..."); channel.lock } else freeLock
+				try { action }
+				finally { lock.release() }
+			}
+			finally { channel.close() }
+		}
+	}
 }
\ No newline at end of file
diff --git a/ivy/IvyCache.scala b/ivy/IvyCache.scala
index 9e9c4d9e4..74c27e3e8 100644
--- a/ivy/IvyCache.scala
+++ b/ivy/IvyCache.scala
@@ -27,6 +27,7 @@ private object NotInCache
 /** Provides methods for working at the level of a single jar file with the default Ivy cache.*/
 object IvyCache
 {
+	def lockFile = new File(System.getProperty("temp.dir"), "sbt.cache.lock")
 	/** Caches the given 'file' with the given ID.  It may be retrieved or cleared using this ID.*/
 	def cacheJar(moduleID: ModuleID, file: File, log: IvyLogger)
 	{
diff --git a/ivy/src/test/scala/ComponentManagerTest.scala b/ivy/src/test/scala/ComponentManagerTest.scala
index 6928ab98a..90613ce1e 100644
--- a/ivy/src/test/scala/ComponentManagerTest.scala
+++ b/ivy/src/test/scala/ComponentManagerTest.scala
@@ -4,47 +4,48 @@ import java.io.File
 import org.specs._
 import FileUtilities.{createDirectory, delete, touch, withTemporaryDirectory}
 import org.apache.ivy.util.ChecksumHelper
+import IfMissing.Fail
 
 object ComponentManagerTest extends Specification
 {
 	val TestID = "manager-test"
 	"Component manager" should {
 		"throw an exception if 'file' is called for a non-existing component" in {
-			withManager { _.file(TestID) must throwA[InvalidComponent] }
+			withManager { _.file(TestID)(Fail) must throwA[InvalidComponent] }
 		}
 		"throw an exception if 'file' is called for an empty component" in {
 			withManager { manager =>
 				manager.define(TestID, Nil)
-				( manager.file(TestID) ) must throwA[InvalidComponent]
+				( manager.file(TestID)(Fail) ) must throwA[InvalidComponent]
 			}
 		}
 		"return the file for a single-file component" in {
 			withManager { manager =>
 				val hash = defineFile(manager, TestID, "a")
-				checksum(manager.file(TestID)) must beEqualTo(hash)
+				checksum(manager.file(TestID)(Fail)) must beEqualTo(hash)
 			}
 		}
 
 		"throw an exception if 'file' is called for multi-file component" in {
 			withManager { manager =>
 				defineFiles(manager, TestID, "a", "b")
-				( manager.file(TestID) ) must throwA[InvalidComponent]
+				( manager.file(TestID)(Fail) ) must throwA[InvalidComponent]
 			}
 		}
 		"return the files for a multi-file component" in {
 			withManager { manager =>
 				val hashes = defineFiles(manager, TestID, "a", "b")
-				checksum(manager.files(TestID)) must haveTheSameElementsAs(hashes)
+				checksum(manager.files(TestID)(Fail)) must haveTheSameElementsAs(hashes)
 			}
 		}
 		"return the files for a single-file component" in {
 			withManager { manager =>
 				val hashes = defineFiles(manager, TestID, "a")
-				checksum(manager.files(TestID)) must haveTheSameElementsAs(hashes)
+				checksum(manager.files(TestID)(Fail)) must haveTheSameElementsAs(hashes)
 			}
 		}
 		"throw an exception if 'files' is called for a non-existing component" in {
-			withManager {   _.files(TestID) must throwA[InvalidComponent] }
+			withManager {   _.files(TestID)(Fail) must throwA[InvalidComponent] }
 		}
 
 		"properly cache a file and then retrieve it to an unresolved component" in {
@@ -54,7 +55,7 @@ object ComponentManagerTest extends Specification
 				{
 					definingManager.cache(TestID)
 					withManager { usingManager =>
-						checksum(usingManager.file(TestID)) must beEqualTo(hash)
+						checksum(usingManager.file(TestID)(Fail)) must beEqualTo(hash)
 					}
 				}
 				finally { definingManager.clearCache(TestID) }
diff --git a/launch/Launch.scala b/launch/Launch.scala
index 8da3ed4fa..b71f41a92 100644
--- a/launch/Launch.scala
+++ b/launch/Launch.scala
@@ -106,4 +106,9 @@ class ComponentProvider(baseDirectory: File) extends xsbti.ComponentProvider
 		else
 			Copy(files, location)
 	}
+	def lockFile =
+	{
+		baseDirectory.mkdirs()
+		new File(baseDirectory, "sbt.components.lock")
+	}
 }
\ No newline at end of file
diff --git a/launch/interface/src/main/java/xsbti/ComponentProvider.java b/launch/interface/src/main/java/xsbti/ComponentProvider.java
index 25d26b803..cac3f9a39 100644
--- a/launch/interface/src/main/java/xsbti/ComponentProvider.java
+++ b/launch/interface/src/main/java/xsbti/ComponentProvider.java
@@ -6,4 +6,5 @@ public interface ComponentProvider
 {
 	public File[] component(String componentID);
 	public void defineComponent(String componentID, File[] components);
+	public File lockFile();
 }
\ No newline at end of file