diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6e41ee9..8ab22f5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -76,6 +76,11 @@
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wshadow")
 endif()
 
+if((CMAKE_COMPILER_IS_GNUCXX AND CMAKE_C_COMPILER_VERSION VERSION_GREATER "4.8.99") OR
+   CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+ set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11 -D_XOPEN_SOURCE=700")
+endif()
+
 add_definitions(-DBORINGSSL_IMPLEMENTATION)
 
 if (BUILD_SHARED_LIBS)
diff --git a/crypto/CMakeLists.txt b/crypto/CMakeLists.txt
index 6433dc6..7430b62 100644
--- a/crypto/CMakeLists.txt
+++ b/crypto/CMakeLists.txt
@@ -145,15 +145,17 @@
   crypto
 
   crypto.c
+  directory_posix.c
+  directory_win.c
+  ex_data.c
   mem.c
+  refcount_c11.c
+  refcount_lock.c
   thread.c
   thread_none.c
   thread_pthread.c
   thread_win.c
-  ex_data.c
   time_support.c
-  directory_posix.c
-  directory_win.c
 
   ${CRYPTO_ARCH_SOURCES}
 
@@ -217,5 +219,13 @@
 
 target_link_libraries(thread_test crypto)
 
+add_executable(
+  refcount_test
+
+  refcount_test.c
+)
+
+target_link_libraries(refcount_test crypto)
+
 perlasm(cpu-x86_64-asm.${ASM_EXT} cpu-x86_64-asm.pl)
 perlasm(cpu-x86-asm.${ASM_EXT} cpu-x86-asm.pl)
diff --git a/crypto/internal.h b/crypto/internal.h
index 42125db..2a11ab5 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -354,6 +354,32 @@
 OPENSSL_EXPORT void CRYPTO_once(CRYPTO_once_t *once, void (*init)(void));
 
 
+/* Reference counting. */
+
+#if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)
+#define OPENSSL_C11_ATOMIC
+#endif
+
+/* CRYPTO_REFCOUNT_MAX is the value at which the reference count saturates. */
+#define CRYPTO_REFCOUNT_MAX 0xffffffff
+
+/* CRYPTO_refcount_inc atomically increments the value at |*count| unless the
+ * value would overflow. It's safe for multiple threads to concurrently call
+ * this or |CRYPTO_refcount_dec_and_test_zero| on the same
+ * |CRYPTO_refcount_t|. */
+OPENSSL_EXPORT void CRYPTO_refcount_inc(CRYPTO_refcount_t *count);
+
+/* CRYPTO_refcount_dec_and_test_zero tests the value at |*count|:
+ *   if it's zero, it crashes the address space.
+ *   if it's the maximum value, it returns zero.
+ *   otherwise, it atomically decrements it and returns one iff the resulting
+ *       value is zero.
+ *
+ * It's safe for multiple threads to concurrently call this or
+ * |CRYPTO_refcount_inc| on the same |CRYPTO_refcount_t|. */
+OPENSSL_EXPORT int CRYPTO_refcount_dec_and_test_zero(CRYPTO_refcount_t *count);
+
+
 /* Locks.
  *
  * Two types of locks are defined: |CRYPTO_MUTEX|, which can be used in
diff --git a/crypto/refcount_c11.c b/crypto/refcount_c11.c
new file mode 100644
index 0000000..a77473f
--- /dev/null
+++ b/crypto/refcount_c11.c
@@ -0,0 +1,65 @@
+/* Copyright (c) 2015, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include "internal.h"
+
+
+#if defined(OPENSSL_C11_ATOMIC)
+
+#include <assert.h>
+#include <stdalign.h>
+#include <stdatomic.h>
+#include <stdlib.h>
+
+#include <openssl/type_check.h>
+
+
+/* See comment above the typedef of CRYPTO_refcount_t about these tests. */
+static_assert(alignof(CRYPTO_refcount_t) == alignof(_Atomic CRYPTO_refcount_t),
+              "_Atomic alters the needed alignment of a reference count");
+static_assert(sizeof(CRYPTO_refcount_t) == sizeof(_Atomic CRYPTO_refcount_t),
+              "_Atomic alters the size of a reference count");
+
+static_assert((CRYPTO_refcount_t)-1 == CRYPTO_REFCOUNT_MAX,
+              "CRYPTO_REFCOUNT_MAX is incorrect");
+
+void CRYPTO_refcount_inc(CRYPTO_refcount_t *count) {
+  uint32_t expected = atomic_load(count);
+
+  while (expected != CRYPTO_REFCOUNT_MAX) {
+    uint32_t new_value = expected + 1;
+    if (atomic_compare_exchange_weak(count, &expected, new_value)) {
+      break;
+    }
+  }
+}
+
+int CRYPTO_refcount_dec_and_test_zero(CRYPTO_refcount_t *count) {
+  uint32_t expected = atomic_load(count);
+
+  for (;;) {
+    if (expected == 0) {
+      abort();
+    } else if (expected == CRYPTO_REFCOUNT_MAX) {
+      return 0;
+    } else {
+      const uint32_t new_value = expected - 1;
+      if (atomic_compare_exchange_weak(count, &expected, new_value)) {
+        return new_value == 0;
+      }
+    }
+  }
+}
+
+#endif  /* OPENSSL_C11_ATOMIC */
diff --git a/crypto/refcount_lock.c b/crypto/refcount_lock.c
new file mode 100644
index 0000000..bb8ef86
--- /dev/null
+++ b/crypto/refcount_lock.c
@@ -0,0 +1,53 @@
+/* Copyright (c) 2015, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include "internal.h"
+
+#include <stdlib.h>
+
+#include <openssl/type_check.h>
+
+
+#if !defined(OPENSSL_C11_ATOMIC)
+
+OPENSSL_COMPILE_ASSERT((CRYPTO_refcount_t)-1 == CRYPTO_REFCOUNT_MAX,
+                       CRYPTO_REFCOUNT_MAX_is_incorrect);
+
+static struct CRYPTO_STATIC_MUTEX g_refcount_lock = CRYPTO_STATIC_MUTEX_INIT;
+
+void CRYPTO_refcount_inc(CRYPTO_refcount_t *count) {
+  CRYPTO_STATIC_MUTEX_lock_write(&g_refcount_lock);
+  if (*count < CRYPTO_REFCOUNT_MAX) {
+    (*count)++;
+  }
+  CRYPTO_STATIC_MUTEX_unlock(&g_refcount_lock);
+}
+
+int CRYPTO_refcount_dec_and_test_zero(CRYPTO_refcount_t *count) {
+  int ret;
+
+  CRYPTO_STATIC_MUTEX_lock_write(&g_refcount_lock);
+  if (*count == 0) {
+    abort();
+  }
+  if (*count < CRYPTO_REFCOUNT_MAX) {
+    (*count)--;
+  }
+  ret = (*count == 0);
+  CRYPTO_STATIC_MUTEX_unlock(&g_refcount_lock);
+
+  return ret;
+}
+
+#endif  /* OPENSSL_C11_ATOMIC */
diff --git a/crypto/refcount_test.c b/crypto/refcount_test.c
new file mode 100644
index 0000000..53db90f
--- /dev/null
+++ b/crypto/refcount_test.c
@@ -0,0 +1,59 @@
+/* Copyright (c) 2015, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
+
+#include "internal.h"
+
+#include <stdio.h>
+
+#include <openssl/type_check.h>
+
+
+int main() {
+  CRYPTO_refcount_t count = 0;
+
+  CRYPTO_refcount_inc(&count);
+  if (count != 1) {
+    fprintf(stderr, "Incrementing reference count did not work.\n");
+    return 1;
+  }
+  if (!CRYPTO_refcount_dec_and_test_zero(&count) || count != 0) {
+    fprintf(stderr, "Decrementing reference count to zero did not work.\n");
+    return 1;
+  }
+
+  count = CRYPTO_REFCOUNT_MAX;
+  CRYPTO_refcount_inc(&count);
+  if (count != CRYPTO_REFCOUNT_MAX) {
+    fprintf(stderr, "Count did not saturate correctly when incrementing.\n");
+    return 1;
+  }
+  if (CRYPTO_refcount_dec_and_test_zero(&count) ||
+      count != CRYPTO_REFCOUNT_MAX) {
+    fprintf(stderr, "Count did not saturate correctly when decrementing.\n");
+    return 1;
+  }
+
+  count = 2;
+  if (CRYPTO_refcount_dec_and_test_zero(&count)) {
+    fprintf(stderr, "Decrementing two resulted in zero!\n");
+    return 1;
+  }
+  if (count != 1) {
+    fprintf(stderr, "Decrementing two did not produce one!");
+    return 1;
+  }
+
+  printf("PASS\n");
+  return 0;
+}
diff --git a/include/openssl/thread.h b/include/openssl/thread.h
index f6e7529..01df6ea 100644
--- a/include/openssl/thread.h
+++ b/include/openssl/thread.h
@@ -90,6 +90,15 @@
 } CRYPTO_MUTEX;
 #endif
 
+/* CRYPTO_refcount_t is the type of a reference count.
+ *
+ * Since some platforms use C11 atomics to access this, it should have the
+ * _Atomic qualifier. However, this header is included by C++ programs as well
+ * as C code that might not set -std=c11. So, in practice, it's not possible to
+ * do that. Instead we statically assert that the size and native alignment of
+ * a plain uint32_t and an _Atomic uint32_t are equal in refcount_c11.c. */
+typedef uint32_t CRYPTO_refcount_t;
+
 
 /* Functions to support multithreading.
  *
diff --git a/util/all_tests.go b/util/all_tests.go
index 91822d1..5b6dd88 100644
--- a/util/all_tests.go
+++ b/util/all_tests.go
@@ -83,6 +83,7 @@
 	{"crypto/lhash/lhash_test"},
 	{"crypto/modes/gcm_test"},
 	{"crypto/pkcs8/pkcs12_test"},
+	{"crypto/refcount_test"},
 	{"crypto/rsa/rsa_test"},
 	{"crypto/thread_test"},
 	{"crypto/x509/pkcs7_test"},