diff --git a/libcxx/lib/abi/x86_64-unknown-linux-gnu.abilist b/libcxx/lib/abi/x86_64-unknown-linux-gnu.abilist
@@ -252,6 +252,7 @@
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNKSt3__121__basic_string_commonILb1EE20__throw_out_of_rangeEv'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNKSt3__123__match_any_but_newlineIcE6__execERNS_7__stateIcEE'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNKSt3__123__match_any_but_newlineIwE6__execERNS_7__stateIwEE'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNKSt3__124__libcpp_debug_exception4whatEv'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNKSt3__15ctypeIcE10do_tolowerEPcPKc'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNKSt3__15ctypeIcE10do_tolowerEc'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNKSt3__15ctypeIcE10do_toupperEPcPKc'}
@@ -1136,6 +1137,16 @@
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__121recursive_timed_mutexD1Ev'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__121recursive_timed_mutexD2Ev'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__121undeclare_no_pointersEPcm'}
+{'is_defined': True, 'type': 'OBJECT', 'name': '_ZNSt3__123__libcpp_debug_functionE', 'size': 8}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionC1ERKNS_19__libcpp_debug_infoE'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionC1ERKS0_'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionC1Ev'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionC2ERKNS_19__libcpp_debug_infoE'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionC2ERKS0_'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionC2Ev'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionD0Ev'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionD1Ev'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__124__libcpp_debug_exceptionD2Ev'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__125__num_get_signed_integralIlEET_PKcS3_Rji'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__125__num_get_signed_integralIxEET_PKcS3_Rji'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__125notify_all_at_thread_exitERNS_18condition_variableENS_11unique_lockINS_5mutexEEE'}
@@ -1154,10 +1165,13 @@
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIwwEEPwEEbT0_S5_T_'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIxxEEPxEEbT0_S5_T_'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__insertion_sort_incompleteIRNS_6__lessIyyEEPyEEbT0_S5_T_'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__libcpp_set_debug_functionEPFvRKNS_19__libcpp_debug_infoEE'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__num_get_unsigned_integralIjEET_PKcS3_Rji'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__num_get_unsigned_integralImEET_PKcS3_Rji'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__num_get_unsigned_integralItEET_PKcS3_Rji'}
 {'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__127__num_get_unsigned_integralIyEET_PKcS3_Rji'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__129__libcpp_abort_debug_functionERKNS_19__libcpp_debug_infoE'}
+{'is_defined': True, 'type': 'FUNC', 'name': '_ZNSt3__129__libcpp_throw_debug_functionERKNS_19__libcpp_debug_infoE'}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZNSt3__13cinE', 'size': 168}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZNSt3__14cerrE', 'size': 160}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZNSt3__14clogE', 'size': 160}
@@ -1554,6 +1568,7 @@
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTINSt3__120__codecvt_utf8_utf16IwEE', 'size': 24}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTINSt3__120__time_get_c_storageIcEE', 'size': 16}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTINSt3__120__time_get_c_storageIwEE', 'size': 16}
+{'is_defined': True, 'type': 'OBJECT', 'name': '_ZTINSt3__124__libcpp_debug_exceptionE', 'size': 24}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTINSt3__15ctypeIcEE', 'size': 56}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTINSt3__15ctypeIwEE', 'size': 56}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTINSt3__16locale5facetE', 'size': 24}
@@ -1679,6 +1694,7 @@
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTSNSt3__120__codecvt_utf8_utf16IwEE', 'size': 34}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTSNSt3__120__time_get_c_storageIcEE', 'size': 34}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTSNSt3__120__time_get_c_storageIwEE', 'size': 34}
+{'is_defined': True, 'type': 'OBJECT', 'name': '_ZTSNSt3__124__libcpp_debug_exceptionE', 'size': 35}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTSNSt3__15ctypeIcEE', 'size': 18}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTSNSt3__15ctypeIwEE', 'size': 18}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTSNSt3__16locale5facetE', 'size': 22}
@@ -1790,6 +1806,7 @@
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTVNSt3__120__codecvt_utf8_utf16IDiEE', 'size': 96}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTVNSt3__120__codecvt_utf8_utf16IDsEE', 'size': 96}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTVNSt3__120__codecvt_utf8_utf16IwEE', 'size': 96}
+{'is_defined': True, 'type': 'OBJECT', 'name': '_ZTVNSt3__124__libcpp_debug_exceptionE', 'size': 40}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTVNSt3__15ctypeIcEE', 'size': 104}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTVNSt3__15ctypeIwEE', 'size': 136}
 {'is_defined': True, 'type': 'OBJECT', 'name': '_ZTVNSt3__16locale5facetE', 'size': 40}

diff --git a/libcxx/src/debug.cpp b/libcxx/src/debug.cpp
@@ -7,16 +7,75 @@
 //
 //===----------------------------------------------------------------------===//
 
-#define _LIBCPP_DEBUG 1
 #include "__config"
 #include "__debug"
 #include "functional"
 #include "algorithm"
+#include "string"
+#include "cstdio"
 #include "__hash_table"
 #include "mutex"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+static std::string make_what_str(__libcpp_debug_info const& info) {
+  string msg = info.__file_;
+  msg += ":" + to_string(info.__line_) + ": _LIBCPP_ASSERT '";
+  msg += info.__pred_;
+  msg += "' failed. ";
+  msg += info.__msg_;
+  return msg;
+}
+
+_LIBCPP_SAFE_STATIC __libcpp_debug_function_type
+    __libcpp_debug_function = __libcpp_abort_debug_function;
+
+bool __libcpp_set_debug_function(__libcpp_debug_function_type __func) {
+  __libcpp_debug_function = __func;
+  return true;
+}
+
+_LIBCPP_NORETURN void __libcpp_abort_debug_function(__libcpp_debug_info const& info) {
+  std::fprintf(stderr, "%s\n", make_what_str(info).c_str());
+  std::abort();
+}
+
+_LIBCPP_NORETURN void __libcpp_throw_debug_function(__libcpp_debug_info const& info) {
+  throw __libcpp_debug_exception(info);
+}
+
+struct __libcpp_debug_exception::__libcpp_debug_exception_imp {
+  __libcpp_debug_info __info_;
+  std::string __what_str_;
+};
+
+__libcpp_debug_exception::__libcpp_debug_exception() _NOEXCEPT
+    : __imp_(nullptr) {
+}
+
+__libcpp_debug_exception::__libcpp_debug_exception(
+    __libcpp_debug_info const& info) : __imp_(new __libcpp_debug_exception_imp)
+{
+  __imp_->__info_ = info;
+  __imp_->__what_str_ = make_what_str(info);
+}
+__libcpp_debug_exception::__libcpp_debug_exception(
+    __libcpp_debug_exception const& other) : __imp_(nullptr) {
+  if (other.__imp_)
+    __imp_ = new __libcpp_debug_exception_imp(*other.__imp_);
+}
+
+__libcpp_debug_exception::~__libcpp_debug_exception() _NOEXCEPT {
+  if (__imp_)
+    delete __imp_;
+}
+
+const char* __libcpp_debug_exception::what() const _NOEXCEPT {
+  if (__imp_)
+    return __imp_->__what_str_.c_str();
+  return "__libcpp_debug_exception";
+}
+
 _LIBCPP_FUNC_VIS
 __libcpp_db*
 __get_db()

diff --git a/libcxx/test/libcxx/debug/debug_abort.pass.cpp b/libcxx/test/libcxx/debug/debug_abort.pass.cpp
@@ -0,0 +1,30 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Test that the default debug handler aborts the program.
+
+#define _LIBCPP_DEBUG 0
+
+#include <csignal>
+#include <cstdlib>
+#include <__debug>
+
+void signal_handler(int signal)
+{
+    if (signal == SIGABRT)
+      std::_Exit(EXIT_SUCCESS);
+    std::_Exit(EXIT_FAILURE);
+}
+
+int main()
+{
+  if (std::signal(SIGABRT, signal_handler) != SIG_ERR)
+    _LIBCPP_ASSERT(false, "foo");
+  return EXIT_FAILURE;
+}
diff --git a/libcxx/test/libcxx/debug/debug_throw.pass.cpp b/libcxx/test/libcxx/debug/debug_throw.pass.cpp
@@ -0,0 +1,36 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: libcpp-no-exceptions
+
+// Test that the default debug handler can be overridden and test the
+// throwing debug handler.
+
+#define _LIBCPP_DEBUG 0
+
+#include <cstdlib>
+#include <exception>
+#include <type_traits>
+#include <__debug>
+
+int main()
+{
+  {
+    std::__libcpp_debug_function = std::__libcpp_throw_debug_function;
+    try {
+      _LIBCPP_ASSERT(false, "foo");
+    } catch (std::__libcpp_debug_exception const&) {}
+  }
+  {
+    // test that the libc++ exception type derives from std::exception
+    static_assert(std::is_base_of<std::exception,
+                                  std::__libcpp_debug_exception
+                  >::value, "must be an exception");
+  }
+}
diff --git a/libcxx/test/libcxx/debug/debug_throw_register.pass.cpp b/libcxx/test/libcxx/debug/debug_throw_register.pass.cpp
@@ -0,0 +1,30 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: libcpp-no-exceptions
+
+// Test that defining _LIBCPP_DEBUG_USE_EXCEPTIONS causes _LIBCPP_ASSERT
+// to throw on failure.
+
+#define _LIBCPP_DEBUG 1
+#define _LIBCPP_DEBUG_USE_EXCEPTIONS
+
+#include <cstdlib>
+#include <exception>
+#include <type_traits>
+#include <__debug>
+#include <cassert>
+
+int main()
+{
+  try {
+    _LIBCPP_ASSERT(false, "foo");
+    assert(false);
+  } catch (...) {}
+}
diff --git a/libcxx/www/debug_mode.html b/libcxx/www/debug_mode.html
diff --git a/libcxx/www/index.html b/libcxx/www/index.html
@@ -220,7 +220,6 @@ <h2>Design Documents</h2>
 <li><a href="atomic_design.html"><tt>&lt;atomic&gt;</tt></a></li>
 <li><a href="type_traits_design.html"><tt>&lt;type_traits&gt;</tt></a></li>
 <li><a href="http://cplusplusmusings.wordpress.com/2012/07/05/clang-and-standard-libraries-on-mac-os-x/">Excellent notes by Marshall Clow</a></li>
-<li><a href="debug_mode.html">Status of debug mode</a></li>
 </ul>
 
 </div>