diff --git a/libcxx/include/__algorithm/pstl_backend.h b/libcxx/include/__algorithm/pstl_backend.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backend.h
@@ -0,0 +1,109 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_PSTL_BACKEND_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKEND_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if defined(_LIBCPP_HAS_PARALLEL_ALGORITHMS) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+/*
+TODO: Documentation of how backends work
+
+A PSTL parallel backend is a type that provides the following static member functions
+at minimum:
+
+  template <class _ExecutionPolicy, class _Iterator, class _BlockFp>
+  static void __for_each(_ExecutionPolicy&&, _Iterator __first, _Iterator __last, _BlockFp __f);
+
+  template <class _ExecutionPolicy, class _Iterator, class _Tp, class _BinOp>
+  static _Tp __reduce(_ExecutionPolicy&&, _Iterator __first, _Iterator __last, _Tp const& __value, _BinOp __op);
+
+  etc...
+
+The following functions are optional but can be provided. If provided, they are used by the corresponding
+algorithms, otherwise they are implemented in terms of the basis operations mentioned above:
+
+  template <class _ExecutionPolicy, class _Iterator, class _Size, class _Fp>
+  static void __for_each_n(_ExecutionPolicy&&, _Iterator __first, _Size __n, _Fp __f);
+
+  etc...
+
+*/
+
+#  if defined(_PSTL_PAR_BACKEND_STD_THREAD)
+#    include <__algorithm/pstl_backends/thread.h>
+using __pstl_par_backend = __std_thread_backend;
+
+#  elif defined(_PSTL_PAR_BACKEND_GCD)
+#    include <__algorithm/pstl_backends/gcd.h>
+using __pstl_par_backend = __gcd_backend;
+
+#  elif defined(_PSTL_PAR_BACKEND_TBB)
+#    include <__algorithm/pstl_backends/tbb.h>
+using __pstl_par_backend = __tbb_backend;
+
+#  elif defined(_PSTL_PAR_BACKEND_SERIAL)
+#    include <__algorithm/pstl_backends/serial.h>
+using __pstl_par_backend = __par_serial_backend;
+
+#  else
+
+// ...New vendors can add parallel backends here...
+
+#    error "Invalid choice of a PSTL parallel backend"
+#  endif
+
+/*
+TODO: Documentation of how backends work
+
+A PSTL unsequenced backend is a type that provides the following static member functions
+at minimum:
+
+  template <class _ExecutionPolicy, class _Iterator, class _Fp>
+  static void ????
+
+  etc...
+
+The following functions are optional but can be provided. If provided, they are used by the corresponding
+algorithms, otherwise they are implemented in terms of the basis operations mentioned above:
+
+  template <class _ExecutionPolicy, class _Iterator, class _Pred>
+  static void ????
+
+  etc...
+
+*/
+
+#  if defined(_PSTL_UNSEQ_BACKEND_SIMD)
+#    include <__algorithm/pstl_unseq_backends/simd.h>
+using __pstl_unseq_backend = __simd_backend;
+
+#  elif defined(_PSTL_UNSEQ_BACKEND_SERIAL)
+#    include <__algorithm/pstl_unseq_backends/serial.h>
+using __pstl_unseq_backend = __unseq_serial_backend;
+
+#  else
+
+// ...New vendors can add unsequenced backends here...
+
+#    error "Invalid choice of a PSTL unsequenced backend"
+#  endif
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // defined(_LIBCPP_HAS_PARALLEL_ALGORITHMS) && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKEND_H
diff --git a/libcxx/include/__algorithm/pstl_backends/serial.h b/libcxx/include/__algorithm/pstl_backends/serial.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backends/serial.h
@@ -0,0 +1,53 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_PSTL_BACKENDS_SERIAL_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_SERIAL_H
+
+#include <__config>
+#include <__algorithm/for_each.h>
+#include <__numeric/reduce.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if defined(_LIBCPP_HAS_PARALLEL_ALGORITHMS) && _LIBCPP_STD_VER >= 17
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+struct __serial_backend {
+    template <class _ExecutionPolicy, class _Iterator, class _Fp>
+    _LIBCPP_HIDE_FROM_ABI
+    static void __for_each(_ExecutionPolicy&&, _Iterator __first, _Iterator __last, _Fp __f) {
+        std::for_each(__first, __last, __f);
+    }
+
+    template <class _ExecutionPolicy, class _Iterator, class _Tp, class _BinOp>
+    _LIBCPP_HIDE_FROM_ABI
+    static _Tp __reduce(_ExecutionPolicy&&, _Iterator __first, _Iterator __last, _Tp const& __value, _BinOp __op) {
+        return std::reduce(__first, __last, __value, __op);
+    }
+
+    // etc...
+
+
+    // Let's say we decide to implement __for_each_n here. It needs to be picked up by std::for_each_n(policy, ...)
+    template <class _ExecutionPolicy, class _Iterator, class _Size, class _Fp>
+    _LIBCPP_HIDE_FROM_ABI
+    static void __for_each_n(_ExecutionPolicy&&, _Iterator __first, _Size __n, _Fp __f) {
+        std::for_each_n(__first, __n, __f);
+    }
+};
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // defined(_LIBCPP_HAS_PARALLEL_ALGORITHMS) && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_SERIAL_H
diff --git a/libcxx/include/__algorithm/pstl_for_each.h b/libcxx/include/__algorithm/pstl_for_each.h
--- a/libcxx/include/__algorithm/pstl_for_each.h
+++ b/libcxx/include/__algorithm/pstl_for_each.h
@@ -9,6 +9,7 @@
 #ifndef _LIBCPP___ALGORITHM_PSTL_FOR_EACH_H
 #define _LIBCPP___ALGORITHM_PSTL_FOR_EACH_H
 
+#include <__algorithm/pstl_backend.h>
 #include <__algorithm/for_each.h>
 #include <__algorithm/for_each_n.h>
 #include <__config>
@@ -32,11 +33,22 @@
           enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI void
 for_each(_ExecutionPolicy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Function __func) {
+  // Possibilities:
+
+  //
+  // 1. Dispatch right here to the backend, let the backend implement the whole thing.
+  //    If we do this, I guess we don't want a separate notion of parallel backend and a
+  //    notion of unsequenced backend?
+  //
+  std::__pstl_backend::__for_each(__policy, __first, __last, __func);
+
+  //
+  // 2. Deconstruct the parallel policy and call the various backends separately.
+  //
   if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
                 __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
     std::__terminate_on_exception([&] {
-      __pstl::__par_backend::__parallel_for(
-          {},
+      std::__pstl_par_backend::__for_each(
           __policy,
           __first,
           __last,
@@ -46,7 +58,7 @@
     });
   } else if constexpr (__is_unsequenced_execution_policy_v<_ExecutionPolicy> &&
                        __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
-    __pstl::__unseq_backend::__simd_walk_1(__first, __last - __first, __func);
+    std::__pstl_unseq_backend::__for_each(__first, __last, __func);
   } else {
     std::for_each(__first, __last, __func);
   }
@@ -59,6 +71,8 @@
           enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI void
 for_each_n(_ExecutionPolicy&& __policy, _ForwardIterator __first, _Size __size, _Function __func) {
+  // TODO: Here we need to detect whether the back-end supports __for_each_n, and if so, call that?
+
   if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
     std::for_each(__policy, __first, __first + __size, __func);
   } else {