diff --git a/Makefile b/Makefile
index 569defc0..06dce5a3 100644
--- a/Makefile
+++ b/Makefile
@@ -234,6 +234,7 @@ TEST_PACKAGES = \
 	os \
 	path \
 	reflect \
+	sync \
 	testing \
 	testing/iotest \
 	text/scanner \
diff --git a/src/sync/cond_test.go b/src/sync/cond_test.go
new file mode 100644
index 00000000..6303ab96
--- /dev/null
+++ b/src/sync/cond_test.go
@@ -0,0 +1,83 @@
+package sync_test
+
+import (
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+// TestCondSignal tests waiting on a Cond and notifying it with Signal.
+func TestCondSignal(t *testing.T) {
+	// Create a Cond with a normal mutex.
+	cond := sync.Cond{
+		L: &sync.Mutex{},
+	}
+	cond.L.Lock()
+
+	// Start a goroutine to signal us once we wait.
+	var signaled uint32
+	go func() {
+		// Wait for the test goroutine to wait.
+		cond.L.Lock()
+		defer cond.L.Unlock()
+
+		// Send a signal to the test goroutine.
+		atomic.StoreUint32(&signaled, 1)
+		cond.Signal()
+	}()
+
+	// Wait for a signal.
+	// This will unlock the mutex, and allow the spawned goroutine to run.
+	cond.Wait()
+	if atomic.LoadUint32(&signaled) == 0 {
+		t.Error("wait returned before a signal was sent")
+	}
+}
+
+func TestCondBroadcast(t *testing.T) {
+	// Create a Cond with an RWMutex.
+	var mu sync.RWMutex
+	cond := sync.Cond{
+		L: mu.RLocker(),
+	}
+
+	// Start goroutines to wait for the broadcast.
+	var wg sync.WaitGroup
+	const n = 5
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+		mu.RLock()
+		go func() {
+			defer wg.Done()
+
+			cond.Wait()
+		}()
+	}
+
+	// Wait for all goroutines to start waiting.
+	mu.Lock()
+
+	// Broadcast to all of the waiting goroutines.
+	cond.Broadcast()
+
+	// Wait for all spawned goroutines to process the broadcast.
+	mu.Unlock()
+	wg.Wait()
+}
+
+// TestCondUnlockNotify verifies that a signal is processed even if it happens during the mutex unlock in Wait.
+func TestCondUnlockNotify(t *testing.T) {
+	// Create a Cond that signals itself when waiting.
+	var cond sync.Cond
+	cond.L = fakeLocker{cond.Signal}
+
+	cond.Wait()
+}
+
+// fakeLocker is a fake sync.Locker where unlock calls an arbitrary function.
+type fakeLocker struct {
+	unlock func()
+}
+
+func (l fakeLocker) Lock()   {}
+func (l fakeLocker) Unlock() { l.unlock() }
diff --git a/src/sync/mutex_test.go b/src/sync/mutex_test.go
new file mode 100644
index 00000000..88ae317d
--- /dev/null
+++ b/src/sync/mutex_test.go
@@ -0,0 +1,205 @@
+package sync_test
+
+import (
+	"runtime"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+// TestMutexUncontended tests locking and unlocking a Mutex that is not shared with any other goroutines.
+func TestMutexUncontended(t *testing.T) {
+	var mu sync.Mutex
+
+	// Lock and unlock the mutex a few times.
+	for i := 0; i < 3; i++ {
+		mu.Lock()
+		mu.Unlock()
+	}
+}
+
+// TestMutexConcurrent tests a mutex concurrently from multiple goroutines.
+// It will fail if multiple goroutines hold the lock simultaneously.
+func TestMutexConcurrent(t *testing.T) {
+	var mu sync.Mutex
+	var active uint
+	var completed uint
+	ok := true
+
+	const n = 10
+	for i := 0; i < n; i++ {
+		j := i
+		go func() {
+			// Delay a bit.
+			for k := j; k > 0; k-- {
+				runtime.Gosched()
+			}
+
+			mu.Lock()
+
+			// Increment the active counter.
+			active++
+
+			if active > 1 {
+				// Multiple things are holding the lock at the same time.
+				ok = false
+			} else {
+				// Delay a bit.
+				for k := j; k < n; k++ {
+					runtime.Gosched()
+				}
+			}
+
+			// Decrement the active counter.
+			active--
+
+			// This is completed.
+			completed++
+
+			mu.Unlock()
+		}()
+	}
+
+	// Wait for everything to finish.
+	var done bool
+	for !done {
+		// Wait a bit for other things to run.
+		runtime.Gosched()
+
+		// Acquire the lock and check whether everything has completed.
+		mu.Lock()
+		done = completed == n
+		mu.Unlock()
+	}
+	if !ok {
+		t.Error("lock held concurrently")
+	}
+}
+
+// TestRWMutexUncontended tests locking and unlocking an RWMutex that is not shared with any other goroutines.
+func TestRWMutexUncontended(t *testing.T) {
+	var mu sync.RWMutex
+
+	// Lock the mutex exclusively and then unlock it.
+	mu.Lock()
+	mu.Unlock()
+
+	// Acuire several read locks.
+	const n = 5
+	for i := 0; i < n; i++ {
+		mu.RLock()
+	}
+
+	// Release all of the read locks.
+	for i := 0; i < n; i++ {
+		mu.RUnlock()
+	}
+
+	// Re-acquire the lock exclusively.
+	mu.Lock()
+	mu.Unlock()
+}
+
+// TestRWMutexWriteToRead tests the transition from a write lock to a read lock while contended.
+func TestRWMutexWriteToRead(t *testing.T) {
+	// Create a new RWMutex and acquire a write lock.
+	var mu sync.RWMutex
+	mu.Lock()
+
+	const n = 3
+	var readAcquires uint32
+	var completed uint32
+	var unlocked uint32
+	var bad uint32
+	for i := 0; i < n; i++ {
+		go func() {
+			// Acquire a read lock.
+			mu.RLock()
+
+			// Verify that the write lock is supposed to be released by now.
+			if atomic.LoadUint32(&unlocked) == 0 {
+				// The write lock is still being held.
+				atomic.AddUint32(&bad, 1)
+			}
+
+			// Add ourselves to the read lock counter.
+			atomic.AddUint32(&readAcquires, 1)
+
+			// Wait for everything to hold the read lock simultaneously.
+			for atomic.LoadUint32(&readAcquires) < n {
+				runtime.Gosched()
+			}
+
+			// Notify of completion.
+			atomic.AddUint32(&completed, 1)
+
+			// Release the read lock.
+			mu.RUnlock()
+		}()
+	}
+
+	// Wait a bit for the goroutines to block.
+	for i := 0; i < 3*n; i++ {
+		runtime.Gosched()
+	}
+
+	// Release the write lock so that the goroutines acquire read locks.
+	atomic.StoreUint32(&unlocked, 1)
+	mu.Unlock()
+
+	// Wait for everything to complete.
+	for atomic.LoadUint32(&completed) < n {
+		runtime.Gosched()
+	}
+
+	// Acquire another write lock.
+	mu.Lock()
+
+	if bad != 0 {
+		t.Error("read lock acquired while write-locked")
+	}
+}
+
+// TestRWMutexWriteToRead tests the transition from a read lock to a write lock while contended.
+func TestRWMutexReadToWrite(t *testing.T) {
+	// Create a new RWMutex and read-lock it several times.
+	const n = 3
+	var mu sync.RWMutex
+	var readers uint32
+	for i := 0; i < n; i++ {
+		mu.RLock()
+		readers++
+	}
+
+	// Start a goroutine to acquire a write lock.
+	result := ^uint32(0)
+	go func() {
+		// Acquire a write lock.
+		mu.Lock()
+
+		// Check for active readers.
+		readers := atomic.LoadUint32(&readers)
+
+		mu.Unlock()
+
+		// Report the number of active readers.
+		atomic.StoreUint32(&result, readers)
+	}()
+
+	// Release the read locks.
+	for i := 0; i < n; i++ {
+		runtime.Gosched()
+		atomic.AddUint32(&readers, ^uint32(0))
+		mu.RUnlock()
+	}
+
+	// Wait for a result.
+	var res uint32
+	for res == ^uint32(0) {
+		runtime.Gosched()
+		res = atomic.LoadUint32(&result)
+	}
+	if res != 0 {
+		t.Errorf("write lock acquired while %d readers were active", res)
+	}
+}
diff --git a/src/sync/once_test.go b/src/sync/once_test.go
new file mode 100644
index 00000000..f6d12d70
--- /dev/null
+++ b/src/sync/once_test.go
@@ -0,0 +1,62 @@
+package sync_test
+
+import (
+	"sync"
+	"testing"
+)
+
+// TestOnceUncontended tests Once on a single goroutine.
+func TestOnceUncontended(t *testing.T) {
+	var once sync.Once
+	{
+		var ran bool
+		once.Do(func() {
+			ran = true
+		})
+		if !ran {
+			t.Error("first call to Do did not run")
+		}
+	}
+	{
+		var ran bool
+		once.Do(func() {
+			ran = true
+		})
+		if ran {
+			t.Error("second call to Do ran")
+		}
+	}
+}
+
+// TestOnceConcurrent tests multiple concurrent invocations of sync.Once.
+func TestOnceConcurrent(t *testing.T) {
+	var once sync.Once
+	var mu sync.Mutex
+	mu.Lock()
+	var ran bool
+	var ranTwice bool
+	once.Do(func() {
+		ran = true
+
+		// Start a goroutine and (approximately) wait for it to enter the call to Do.
+		var startWait sync.Mutex
+		startWait.Lock()
+		go func() {
+			startWait.Unlock()
+			once.Do(func() {
+				ranTwice = true
+			})
+			mu.Unlock()
+		}()
+		startWait.Lock()
+	})
+	if !ran {
+		t.Error("first call to Do did not run")
+	}
+
+	// Wait for the goroutine to finish.
+	mu.Lock()
+	if ranTwice {
+		t.Error("second concurrent call to Once also ran")
+	}
+}
diff --git a/src/sync/waitgroup_test.go b/src/sync/waitgroup_test.go
new file mode 100644
index 00000000..a808f7e1
--- /dev/null
+++ b/src/sync/waitgroup_test.go
@@ -0,0 +1,35 @@
+package sync_test
+
+import (
+	"sync"
+	"testing"
+)
+
+// TestWaitGroupUncontended tests the wait group from a single goroutine.
+func TestWaitGroupUncontended(t *testing.T) {
+	// Check that a single add-and-done works.
+	var wg sync.WaitGroup
+	wg.Add(1)
+	wg.Done()
+	wg.Wait()
+
+	// Check that mixing positive and negative counts works.
+	wg.Add(10)
+	wg.Add(-8)
+	wg.Add(-1)
+	wg.Add(0)
+	wg.Done()
+	wg.Wait()
+}
+
+// TestWaitGroup tests the typical usage of WaitGroup.
+func TestWaitGroup(t *testing.T) {
+	const n = 5
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go wg.Done()
+	}
+
+	wg.Wait()
+}