diff --git a/src/system_wrappers/source/critical_section_unittest.cc b/src/system_wrappers/source/critical_section_unittest.cc
new file mode 100644
index 000000000..c48b9f751
--- /dev/null
+++ b/src/system_wrappers/source/critical_section_unittest.cc
@@ -0,0 +1,167 @@
+/*
+ *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifdef _WIN32
+// For Sleep()
+#include <windows.h>
+#else
+// For nanosleep()
+#include <time.h>
+#endif
+
+#include "system_wrappers/interface/critical_section_wrapper.h"
+
+#include "gtest/gtest.h"
+#include "system_wrappers/interface/sleep.h"
+#include "system_wrappers/interface/thread_wrapper.h"
+#include "system_wrappers/interface/trace.h"
+#include "system_wrappers/source/unittest_utilities.h"
+
+namespace webrtc {
+
+namespace {
+
+const bool kLogTrace = false;  // Set to true to enable debug logging to stdout.
+
+#define LOG(...) WEBRTC_TRACE(kTraceStateInfo, kTraceUtility, -1, __VA_ARGS__);
+
+// Cause a process switch. Needed to avoid depending on
+// busy-wait in tests.
+static void SwitchProcess() {
+  // Note - sched_yield has been tried as process switch. This does
+  // not cause a process switch enough of the time for reliability.
+  SleepMs(1);
+}
+
+class ProtectedCount {
+ public:
+  explicit ProtectedCount(CriticalSectionWrapper* crit_sect)
+    : crit_sect_(crit_sect),
+      count_(0) {
+  }
+
+  void Increment() {
+    CriticalSectionScoped cs(crit_sect_);
+    ++count_;
+    LOG("Inc to %d", count_);
+  }
+
+  int Count() const {
+    CriticalSectionScoped cs(crit_sect_);
+    return count_;
+  }
+
+ private:
+  CriticalSectionWrapper* crit_sect_;
+  int count_;
+};
+
+class CritSectTest : public ::testing::Test {
+ public:
+  CritSectTest() : trace_(kLogTrace) {
+  }
+
+  // Waits a number of cycles for the count to reach a given value.
+  // Returns true if the target is reached or passed.
+  bool WaitForCount(int target, ProtectedCount* count) {
+    int loop_counter = 0;
+    // On Posix, this SwitchProcess() needs to be in a loop to make the
+    // test both fast and non-flaky.
+    // With 1 us wait as the switch, up to 7 rounds have been observed.
+    while (count->Count() < target && loop_counter < 100*target) {
+      ++loop_counter;
+      SwitchProcess();
+    }
+    LOG("Test looped %d times\n", loop_counter);
+    return (count->Count() >= target);
+  }
+
+ private:
+  ScopedTracing trace_;
+};
+
+bool LockUnlockThenStopRunFunction(void* obj) {
+  LOG("Wait starting");
+  ProtectedCount* the_count = static_cast<ProtectedCount*> (obj);
+  LOG("Wait incrementing");
+  the_count->Increment();
+  LOG("Wait returning");
+  return false;
+}
+
+TEST_F(CritSectTest, ThreadWakesOnce) {
+  CriticalSectionWrapper* crit_sect
+      = CriticalSectionWrapper::CreateCriticalSection();
+  ProtectedCount count(crit_sect);
+  ThreadWrapper* thread = ThreadWrapper::CreateThread(
+      &LockUnlockThenStopRunFunction, &count);
+  unsigned int id = 42;
+  crit_sect->Enter();
+  ASSERT_TRUE(thread->Start(id));
+  SwitchProcess();
+  // The critical section is of reentrant mode, so this should not release
+  // the lock, even though count.Count() locks and unlocks the critical section
+  // again.
+  // Thus, the thread should not be able to increment the count
+  ASSERT_EQ(0, count.Count());
+  crit_sect->Leave();  // This frees the thread to act.
+  EXPECT_TRUE(WaitForCount(1, &count));
+  EXPECT_TRUE(thread->Stop());
+  delete thread;
+  delete crit_sect;
+}
+
+bool LockUnlockRunFunction(void* obj) {
+  LOG("Wait starting");
+  ProtectedCount* the_count = static_cast<ProtectedCount*> (obj);
+  LOG("Wait incrementing");
+  the_count->Increment();
+  SwitchProcess();
+  LOG("Wait returning");
+  return true;
+}
+
+TEST_F(CritSectTest, ThreadWakesTwice) {
+  CriticalSectionWrapper* crit_sect
+      = CriticalSectionWrapper::CreateCriticalSection();
+  ProtectedCount count(crit_sect);
+  ThreadWrapper* thread = ThreadWrapper::CreateThread(&LockUnlockRunFunction,
+                                                      &count);
+  unsigned int id = 42;
+  crit_sect->Enter();  // Make sure counter stays 0 until we wait for it.
+  ASSERT_TRUE(thread->Start(id));
+  crit_sect->Leave();
+
+  // The thread is capable of grabbing the lock multiple times,
+  // incrementing counter once each time.
+  // It's possible for the count to be incremented by more than 2.
+  EXPECT_TRUE(WaitForCount(2, &count));
+  EXPECT_LE(2, count.Count());
+
+  // The thread does not increment while lock is held.
+  crit_sect->Enter();
+  int count_before = count.Count();
+  for (int i = 0; i < 10; i++) {
+    SwitchProcess();
+  }
+  EXPECT_EQ(count_before, count.Count());
+  crit_sect->Leave();
+
+  thread->SetNotAlive();  // Tell thread to exit once run function finishes.
+  SwitchProcess();
+  EXPECT_LT(count_before, count.Count());
+  EXPECT_TRUE(thread->Stop());
+  delete thread;
+  delete crit_sect;
+}
+
+}  // anonymous namespace
+
+}  // namespace webrtc
diff --git a/src/system_wrappers/source/system_wrappers.gyp b/src/system_wrappers/source/system_wrappers.gyp
index f8a1fcf92..bda3494a0 100644
--- a/src/system_wrappers/source/system_wrappers.gyp
+++ b/src/system_wrappers/source/system_wrappers.gyp
@@ -189,6 +189,7 @@
             'cpu_wrapper_unittest.cc',
             'cpu_measurement_harness.h',
             'cpu_measurement_harness.cc',
+            'critical_section_unittest.cc',
             'list_unittest.cc',
             'map_unittest.cc',
             'data_log_unittest.cc',