diff --git a/include/boost/mp11/algorithm.hpp b/include/boost/mp11/algorithm.hpp
index 4e4ff7f..6ffc572 100644
--- a/include/boost/mp11/algorithm.hpp
+++ b/include/boost/mp11/algorithm.hpp
@@ -519,6 +519,19 @@ template<class L, class V> using mp_remove = typename detail::mp_remove_impl<L,
 // mp_remove_if<L, P>
 //   in detail/mp_remove_if.hpp
 
+// mp_flatten<L, L2 = mp_clear<L>>
+namespace detail
+{
+
+template<class L2> struct mp_flatten_impl
+{
+    template<class T> using fn = mp_if<mp_similar<L2, T>, T, mp_list<T>>;
+};
+
+} // namespace detail
+
+template<class L, class L2 = mp_clear<L>> using mp_flatten = mp_apply<mp_append, mp_push_front<mp_transform_q<detail::mp_flatten_impl<L2>, L>, mp_clear<L>>>;
+
 // mp_partition<L, P>
 namespace detail
 {
diff --git a/test/Jamfile b/test/Jamfile
index 542221f..440bd9e 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -116,6 +116,7 @@ run mp_nth_element.cpp ;
 run mp_nth_element_q.cpp ;
 run mp_back.cpp ;
 run mp_pop_back.cpp ;
+run mp_flatten.cpp ;
 
 # integral
 run integral.cpp ;
diff --git a/test/mp_flatten.cpp b/test/mp_flatten.cpp
new file mode 100644
index 0000000..403b870
--- /dev/null
+++ b/test/mp_flatten.cpp
@@ -0,0 +1,101 @@
+
+// Copyright 2019 Peter Dimov.
+//
+// Distributed under the Boost Software License, Version 1.0.
+//
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+#include <tuple>
+
+int main()
+{
+    using boost::mp11::mp_list;
+    using boost::mp11::mp_flatten;
+    using boost::mp11::mp_transform;
+
+    {
+        using L1 = mp_list<>;
+        using L2 = mp_list<void>;
+        using L3 = mp_list<void, void>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L1>, L1>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L2>, L2>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L3>, L3>));
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L1, L1>, L1>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L2, L2>, L2>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L3, L3>, L3>));
+
+        using L4 = mp_transform<mp_list, L3>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L4>, L3>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L4, mp_list<>>, L3>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L4, std::tuple<>>, L4>));
+
+        using L5 = mp_transform<std::tuple, L3>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L5>, L5>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L5, mp_list<>>, L5>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L5, std::tuple<>>, L3>));
+    }
+
+    {
+        using L1 = std::tuple<>;
+        using L2 = std::tuple<void>;
+        using L3 = std::tuple<void, void>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L1>, L1>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L2>, L2>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L3>, L3>));
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L1, L1>, L1>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L2, L2>, L2>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L3, L3>, L3>));
+
+        using L4 = mp_transform<mp_list, L3>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L4>, L4>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L4, mp_list<>>, L3>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L4, std::tuple<>>, L4>));
+
+        using L5 = mp_transform<std::tuple, L3>;
+
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L5>, L3>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L5, mp_list<>>, L5>));
+        BOOST_TEST_TRAIT_TRUE((std::is_same<mp_flatten<L5, std::tuple<>>, L3>));
+    }
+
+    {
+        using L1 = mp_list<std::tuple<>, int, mp_list<>, void, mp_list<char, double>, std::pair<int, void>>;
+
+        using R1 = mp_flatten<L1>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R1, mp_list<std::tuple<>, int, void, char, double, std::pair<int, void>>>));
+
+        using R2 = mp_flatten<L1, std::pair<void, void>>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R2, mp_list<std::tuple<>, int, mp_list<>, void, mp_list<char, double>, int, void>>));
+
+        using R3 = mp_flatten<L1, std::tuple<>>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R3, mp_list<int, mp_list<>, void, mp_list<char, double>, std::pair<int, void>>>));
+    }
+
+    {
+        using L1 = std::tuple<std::tuple<>, int, mp_list<>, void, mp_list<char, double>, std::pair<int, void>>;
+
+        using R1 = mp_flatten<L1, mp_list<void, void, void, void>>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R1, std::tuple<std::tuple<>, int, void, char, double, std::pair<int, void>>>));
+
+        using R2 = mp_flatten<L1, std::pair<void, void>>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R2, std::tuple<std::tuple<>, int, mp_list<>, void, mp_list<char, double>, int, void>>));
+
+        using R3 = mp_flatten<L1>;
+        BOOST_TEST_TRAIT_TRUE((std::is_same<R3, std::tuple<int, mp_list<>, void, mp_list<char, double>, std::pair<int, void>>>));
+    }
+
+    return boost::report_errors();
+}