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 +1,91 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <__algorithm/pstl_backends/serial.h>
+#include <__algorithm/pstl_backends/simd.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 tag type to which the following functions are associated, at minimum:
+
+  template <class _ExecutionPolicy, class _Iterator, class _Func>
+  void __for_each(_Backend, _ExecutionPolicy&&, _Iterator __first, _Iterator __last, _Func __f);
+
+  template <class _ExecutionPolicy, class _Iterator, class _Tp, class _BinOp>
+  _Tp __reduce(_Backend, _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 _Func>
+  void __for_each_n(_Backend, _ExecutionPolicy&&, _Iterator __first, _Size __n, _Func __f);
+
+  etc...
+
+*/
+
+template <class _ExecutionPolicy>
+struct __select_backend;
+
+template <>
+struct __select_backend<std::sequenced_policy> {
+  using type = __serial_backend;
+};
+
+template <>
+struct __select_backend<std::unsequenced_policy> {
+  using type = __simd_backend;
+};
+
+#  if defined(_PSTL_PAR_BACKEND_STD_THREAD)
+#    include <__algorithm/pstl_backends/thread.h>
+template <> struct __select_backend<std::parallel_policy> { using type = __std_thread_backend; };
+template <> struct __select_backend<std::parallel_unsequenced_policy> { using type = __std_thread_backend; };
+
+#  elif defined(_PSTL_PAR_BACKEND_GCD)
+#    include <__algorithm/pstl_backends/gcd.h>
+template <> struct __select_backend<std::parallel_policy> { using type = __gcd_backend; };
+template <> struct __select_backend<std::parallel_unsequenced_policy> { using type = __gcd_backend; };
+
+#  elif defined(_PSTL_PAR_BACKEND_TBB)
+#    include <__algorithm/pstl_backends/tbb.h>
+template <> struct __select_backend<std::parallel_policy> { using type = __tbb_backend; };
+template <> struct __select_backend<std::parallel_unsequenced_policy> { using type = __tbb_backend; };
+
+#  elif defined(_PSTL_PAR_BACKEND_SERIAL)
+#    include <__algorithm/pstl_backends/serial.h>
+template <> struct __select_backend<std::parallel_policy> { using type = __serial_backend; };
+template <> struct __select_backend<std::parallel_unsequenced_policy> { using type = __serial_backend; };
+
+#  else
+
+// ...New vendors can add parallel backends here...
+
+#    error "Invalid choice of a PSTL parallel 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 +1,57 @@
+// -*- 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 { };
+
+//
+// Mandatory customization points
+//
+template <class _ExecutionPolicy, class _Iterator, class _Fp>
+_LIBCPP_HIDE_FROM_ABI
+void __pstl_for_each(__serial_backend, _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
+_Tp __pstl_reduce(__serial_backend, _Iterator __first, _Iterator __last, _Tp const& __value, _BinOp __op) {
+    return std::reduce(__first, __last, __value, __op);
+}
+
+// etc...
+
+//
+// Optional customization points
+//
+template <class _ExecutionPolicy, class _Iterator, class _Size, class _Fp>
+_LIBCPP_HIDE_FROM_ABI
+void __pstl_for_each_n(__serial_backend, _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_backends/thread.h b/libcxx/include/__algorithm/pstl_backends/thread.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_backends/thread.h
@@ -0,0 +1,57 @@
+// -*- 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_THREAD_H
+#define _LIBCPP___ALGORITHM_PSTL_BACKENDS_THREAD_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 __thread_backend { };
+
+//
+// Mandatory customization points
+//
+template <class _ExecutionPolicy, class _Iterator, class _Fp>
+_LIBCPP_HIDE_FROM_ABI
+void __pstl_for_each(__thread_backend, _Iterator __first, _Iterator __last, _Fp __f) {
+    static_assert(__is_parallel_policy_v<_ExecutionPolicy>);
+    auto& __thread_pool = __get_thread_pool();
+    std::__terminate_on_exception([&] {
+        // chunk up the range and dispatch it to the threads
+        for (TODO) {
+            __thread_pool.__dispatch([&] {
+                __simd_utils::__for_each<_ExecutionPolicy>(__brick_first, __brick_last, __f);
+            });
+        }
+    });
+}
+
+template <class _ExecutionPolicy, class _Iterator, class _Tp, class _BinOp>
+_LIBCPP_HIDE_FROM_ABI
+_Tp __pstl_reduce(__thread_backend, _Iterator __first, _Iterator __last, _Tp const& __value, _BinOp __op) {
+    // TODO: Implement
+}
+
+// etc...
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // defined(_LIBCPP_HAS_PARALLEL_ALGORITHMS) && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_BACKENDS_THREAD_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>
@@ -30,40 +31,33 @@
 template <class _ExecutionPolicy,
           class _ForwardIterator,
           class _Function,
-          enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
+          class _RawPolicy = __remove_cvref_t<_ExecutionPolicy>,
+          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI void
 for_each(_ExecutionPolicy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Function __func) {
-  if constexpr (__is_parallel_execution_policy_v<_ExecutionPolicy> &&
-                __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
-    std::__terminate_on_exception([&] {
-      __pstl::__par_backend::__parallel_for(
-          {},
-          __policy,
-          __first,
-          __last,
-          [&__policy, __func](_ForwardIterator __brick_first, _ForwardIterator __brick_last) {
-            std::for_each(std::__remove_parallel_policy(__policy), __brick_first, __brick_last, __func);
-          });
-    });
-  } 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);
-  } else {
-    std::for_each(__first, __last, __func);
-  }
+  using _Backend = typename __select_backend<_RawPolicy>::type;
+  __pstl_for_each<_RawPolicy>(_Backend{}, std::move(__first), std::move(__last), std::move(__func)); // TODO: Can't do ADL
 }
 
 template <class _ExecutionPolicy,
           class _ForwardIterator,
           class _Size,
           class _Function,
-          enable_if_t<is_execution_policy_v<__remove_cvref_t<_ExecutionPolicy>>, int> = 0>
+          class _RawPolicy = __remove_cvref_t<_ExecutionPolicy>,
+          enable_if_t<is_execution_policy_v<_RawPolicy>, int> = 0>
 _LIBCPP_HIDE_FROM_ABI void
 for_each_n(_ExecutionPolicy&& __policy, _ForwardIterator __first, _Size __size, _Function __func) {
-  if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
-    std::for_each(__policy, __first, __first + __size, __func);
+  using _Backend = typename __select_backend<_RawPolicy>::type;
+  if constexpr (HAS-FOR_EACH_N(_Backend)) {
+    __pstl_for_each_n<_RawPolicy>(_Backend{}, std::move(__first), __size, std::move(__func));
   } else {
-    std::for_each_n(__first, __size, __func);
+    // Default implementation
+    if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value &&
+                  __is_parallel_execution_policy_v<_RawPolicy>) {
+      std::for_each(__policy, __first, __first + __size, __func);
+    } else {
+      std::for_each_n(__first, __size, __func);
+    }
   }
 }
 
diff --git a/libcxx/include/__algorithm/pstl_simd_utils.h b/libcxx/include/__algorithm/pstl_simd_utils.h
new file mode 100644
--- /dev/null
+++ b/libcxx/include/__algorithm/pstl_simd_utils.h
@@ -0,0 +1,50 @@
+// -*- 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_SIMD_UTILS_H
+#define _LIBCPP___ALGORITHM_PSTL_SIMD_UTILS_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
+
+namespace __simd_utils {
+  template <class _ExecutionPolicy, class _ForwardIterator, class _Func>
+  _LIBCPP_HIDE_FROM_ABI
+  void __for_each(_ForwardIterator __first, _ForwardIterator __last, _Func __f) {
+    if constexpr (__is_unsequenced_policy_v<_ExecutionPolicy> &&
+                  __is_cpp17_random_access_iterator<_ForwardIterator>::value) {
+        __simd_walk_1(__first, __last - __first, __f);
+    } else {
+        std::for_each(__first, __last, __f);
+    }
+  }
+
+  template <class _ExecutionPolicy, class _Iterator, class _Tp, class _BinOp>
+  _LIBCPP_HIDE_FROM_ABI
+  _Tp __reduce(_Iterator __first, _Iterator __last, _Tp const& __value, _BinOp __op) {
+      // TODO: Implement
+  }
+}
+
+// etc...
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // defined(_LIBCPP_HAS_PARALLEL_ALGORITHMS) && _LIBCPP_STD_VER >= 17
+
+#endif // _LIBCPP___ALGORITHM_PSTL_SIMD_UTILS_H