diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index ea1b5e4..918680d 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -17,6 +17,7 @@
         "enum_test.cpp",
         "fake_guard_test.cpp",
         "flags_test.cpp",
+        "function_test.cpp",
         "future_test.cpp",
         "match_test.cpp",
         "mixins_test.cpp",
diff --git a/libs/ftl/function_test.cpp b/libs/ftl/function_test.cpp
new file mode 100644
index 0000000..91b5e08
--- /dev/null
+++ b/libs/ftl/function_test.cpp
@@ -0,0 +1,379 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <ftl/function.h>
+#include <gtest/gtest.h>
+
+#include <array>
+#include <cstddef>
+#include <cstdint>
+#include <string_view>
+#include <type_traits>
+
+namespace android::test {
+namespace {
+
+// Create an alias to composite requirements defined by the trait class `T` for easier testing.
+template <typename T, typename S>
+inline constexpr bool is_opaquely_storable = (T::template require_trivially_copyable<S> &&
+                                              T::template require_trivially_destructible<S> &&
+                                              T::template require_will_fit_in_opaque_storage<S> &&
+                                              T::template require_alignment_compatible<S>);
+
+// `I` gives a count of sizeof(std::intptr_t) bytes , and `J` gives a raw count of bytes
+template <size_t I, size_t J = 0>
+struct KnownSizeFunctionObject {
+  using Data = std::array<std::byte, sizeof(std::intptr_t) * I + J>;
+  void operator()() const {};
+  Data data{};
+};
+
+}  // namespace
+
+// static_assert the expected type traits
+static_assert(std::is_invocable_r_v<void, ftl::Function<void()>>);
+static_assert(std::is_trivially_copyable_v<ftl::Function<void()>>);
+static_assert(std::is_trivially_destructible_v<ftl::Function<void()>>);
+static_assert(std::is_trivially_copy_constructible_v<ftl::Function<void()>>);
+static_assert(std::is_trivially_move_constructible_v<ftl::Function<void()>>);
+static_assert(std::is_trivially_copy_assignable_v<ftl::Function<void()>>);
+static_assert(std::is_trivially_move_assignable_v<ftl::Function<void()>>);
+
+template <typename T>
+using function_traits = ftl::details::function_traits<T>;
+
+// static_assert that the expected value of N is used for known function object sizes.
+static_assert(function_traits<KnownSizeFunctionObject<0, 0>>::size == 0);
+static_assert(function_traits<KnownSizeFunctionObject<0, 1>>::size == 0);
+static_assert(function_traits<KnownSizeFunctionObject<1, 0>>::size == 0);
+static_assert(function_traits<KnownSizeFunctionObject<1, 1>>::size == 1);
+static_assert(function_traits<KnownSizeFunctionObject<2, 0>>::size == 1);
+static_assert(function_traits<KnownSizeFunctionObject<2, 1>>::size == 2);
+
+// Check that is_function_v works
+static_assert(!ftl::is_function_v<KnownSizeFunctionObject<0>>);
+static_assert(!ftl::is_function_v<std::function<void()>>);
+static_assert(ftl::is_function_v<ftl::Function<void()>>);
+
+// static_assert what can and cannot be stored inside the opaque storage
+
+template <size_t N>
+using function_opaque_storage = ftl::details::function_opaque_storage<N>;
+
+// Function objects can be stored if they fit.
+static_assert(is_opaquely_storable<function_opaque_storage<0>, KnownSizeFunctionObject<0>>);
+static_assert(is_opaquely_storable<function_opaque_storage<0>, KnownSizeFunctionObject<1>>);
+static_assert(!is_opaquely_storable<function_opaque_storage<0>, KnownSizeFunctionObject<2>>);
+
+static_assert(is_opaquely_storable<function_opaque_storage<1>, KnownSizeFunctionObject<2>>);
+static_assert(!is_opaquely_storable<function_opaque_storage<1>, KnownSizeFunctionObject<3>>);
+
+static_assert(is_opaquely_storable<function_opaque_storage<2>, KnownSizeFunctionObject<3>>);
+static_assert(!is_opaquely_storable<function_opaque_storage<2>, KnownSizeFunctionObject<4>>);
+
+// Another opaque storage can be stored if it fits. This property is used to copy smaller
+// ftl::Functions into larger ones.
+static_assert(is_opaquely_storable<function_opaque_storage<2>, function_opaque_storage<0>::type>);
+static_assert(is_opaquely_storable<function_opaque_storage<2>, function_opaque_storage<1>::type>);
+static_assert(is_opaquely_storable<function_opaque_storage<2>, function_opaque_storage<2>::type>);
+static_assert(!is_opaquely_storable<function_opaque_storage<2>, function_opaque_storage<3>::type>);
+
+// Function objects that aren't trivially copyable or destroyable cannot be stored.
+auto lambda_capturing_unique_ptr = [ptr = std::unique_ptr<void*>()] { static_cast<void>(ptr); };
+static_assert(
+    !is_opaquely_storable<function_opaque_storage<2>, decltype(lambda_capturing_unique_ptr)>);
+
+// Keep in sync with "Example usage" in header file.
+TEST(Function, Example) {
+  using namespace std::string_view_literals;
+
+  class MyClass {
+   public:
+    void on_event() const {}
+    int on_string(int*, std::string_view) { return 1; }
+
+    auto get_function() {
+      return ftl::make_function([this] { on_event(); });
+    }
+  } cls;
+
+  // A function container with no arguments, and returning no value.
+  ftl::Function<void()> f;
+
+  // Construct a ftl::Function containing a small lambda.
+  f = cls.get_function();
+
+  // Construct a ftl::Function that calls `cls.on_event()`.
+  f = ftl::make_function<&MyClass::on_event>(&cls);
+
+  // Create a do-nothing function.
+  f = ftl::no_op;
+
+  // Invoke the contained function.
+  f();
+
+  // Also invokes it.
+  std::invoke(f);
+
+  // Create a typedef to give a more meaningful name and bound the size.
+  using MyFunction = ftl::Function<int(std::string_view), 2>;
+  int* ptr = nullptr;
+  auto f1 =
+      MyFunction::make([cls = &cls, ptr](std::string_view sv) { return cls->on_string(ptr, sv); });
+  int r = f1("abc"sv);
+
+  // Returns a default-constructed int (0).
+  f1 = ftl::no_op;
+  r = f1("abc"sv);
+  EXPECT_EQ(r, 0);
+}
+
+TEST(Function, BasicOperations) {
+  // Default constructible.
+  ftl::Function<int()> f;
+
+  // Compares as empty
+  EXPECT_FALSE(f);
+  EXPECT_TRUE(f == nullptr);
+  EXPECT_FALSE(f != nullptr);
+  EXPECT_TRUE(ftl::Function<int()>() == f);
+  EXPECT_FALSE(ftl::Function<int()>() != f);
+
+  // Assigning no_op sets it to not empty.
+  f = ftl::no_op;
+
+  // Verify it can be called, and that it returns a default constructed value.
+  EXPECT_EQ(f(), 0);
+
+  // Comparable when non-empty.
+  EXPECT_TRUE(f);
+  EXPECT_FALSE(f == nullptr);
+  EXPECT_TRUE(f != nullptr);
+  EXPECT_FALSE(ftl::Function<int()>() == f);
+  EXPECT_TRUE(ftl::Function<int()>() != f);
+
+  // Constructing from nullptr means empty.
+  f = ftl::Function<int()>{nullptr};
+  EXPECT_FALSE(f);
+
+  // Assigning nullptr means it is empty.
+  f = nullptr;
+  EXPECT_FALSE(f);
+
+  // Move construction
+  f = ftl::no_op;
+  ftl::Function<int()> g{std::move(f)};
+  EXPECT_TRUE(g != nullptr);
+
+  // Move assignment
+  f = nullptr;
+  f = std::move(g);
+  EXPECT_TRUE(f != nullptr);
+
+  // Copy construction
+  ftl::Function<int()> h{f};
+  EXPECT_TRUE(h != nullptr);
+
+  // Copy assignment
+  g = h;
+  EXPECT_TRUE(g != nullptr);
+}
+
+TEST(Function, CanMoveConstructFromLambda) {
+  auto lambda = [] {};
+  ftl::Function<void()> f{std::move(lambda)};
+}
+
+TEST(Function, TerseDeducedConstructAndAssignFromLambda) {
+  auto f = ftl::Function([] { return 1; });
+  EXPECT_EQ(f(), 1);
+
+  f = [] { return 2; };
+  EXPECT_EQ(f(), 2);
+}
+
+namespace {
+
+struct ImplicitConversionsHelper {
+  auto exact(int) -> int { return 0; }
+  auto inexact(long) -> short { return 0; }
+  // TODO: Switch to `auto templated(auto x)` with C++20
+  template <typename T>
+  T templated(T x) {
+    return x;
+  }
+
+  static auto static_exact(int) -> int { return 0; }
+  static auto static_inexact(long) -> short { return 0; }
+  // TODO: Switch to `static auto static_templated(auto x)` with C++20
+  template <typename T>
+  static T static_templated(T x) {
+    return x;
+  }
+};
+
+}  // namespace
+
+TEST(Function, ImplicitConversions) {
+  using Function = ftl::Function<int(int)>;
+  auto check = [](Function f) { return f(0); };
+  auto exact = [](int) -> int { return 0; };
+  auto inexact = [](long) -> short { return 0; };
+  auto templated = [](auto x) { return x; };
+
+  ImplicitConversionsHelper helper;
+
+  // Note, `check(nullptr)` would crash, so we can only check if it would be invocable.
+  static_assert(std::is_invocable_v<decltype(check), decltype(nullptr)>);
+
+  // Note: We invoke each of these to fully expand all the templates involved.
+  EXPECT_EQ(check(ftl::no_op), 0);
+
+  EXPECT_EQ(check(exact), 0);
+  EXPECT_EQ(check(inexact), 0);
+  EXPECT_EQ(check(templated), 0);
+
+  EXPECT_EQ(check(Function::make<&ImplicitConversionsHelper::exact>(&helper)), 0);
+  EXPECT_EQ(check(Function::make<&ImplicitConversionsHelper::inexact>(&helper)), 0);
+  EXPECT_EQ(check(Function::make<&ImplicitConversionsHelper::templated<int>>(&helper)), 0);
+
+  EXPECT_EQ(check(Function::make<&ImplicitConversionsHelper::static_exact>()), 0);
+  EXPECT_EQ(check(Function::make<&ImplicitConversionsHelper::static_inexact>()), 0);
+  EXPECT_EQ(check(Function::make<&ImplicitConversionsHelper::static_templated<int>>()), 0);
+}
+
+TEST(Function, MakeWithNonConstMemberFunction) {
+  struct Observer {
+    bool called = false;
+    void setCalled() { called = true; }
+  } observer;
+
+  auto f = ftl::make_function<&Observer::setCalled>(&observer);
+
+  f();
+
+  EXPECT_TRUE(observer.called);
+
+  EXPECT_TRUE(f == ftl::Function<void()>::make<&Observer::setCalled>(&observer));
+}
+
+TEST(Function, MakeWithConstMemberFunction) {
+  struct Observer {
+    mutable bool called = false;
+    void setCalled() const { called = true; }
+  } observer;
+
+  const auto f = ftl::make_function<&Observer::setCalled>(&observer);
+
+  f();
+
+  EXPECT_TRUE(observer.called);
+
+  EXPECT_TRUE(f == ftl::Function<void()>::make<&Observer::setCalled>(&observer));
+}
+
+TEST(Function, MakeWithConstClassPointer) {
+  const struct Observer {
+    mutable bool called = false;
+    void setCalled() const { called = true; }
+  } observer;
+
+  const auto f = ftl::make_function<&Observer::setCalled>(&observer);
+
+  f();
+
+  EXPECT_TRUE(observer.called);
+
+  EXPECT_TRUE(f == ftl::Function<void()>::make<&Observer::setCalled>(&observer));
+}
+
+TEST(Function, MakeWithNonCapturingLambda) {
+  auto f = ftl::make_function([](int a, int b) { return a + b; });
+  EXPECT_EQ(f(1, 2), 3);
+}
+
+TEST(Function, MakeWithCapturingLambda) {
+  bool called = false;
+  auto f = ftl::make_function([&called](int a, int b) {
+    called = true;
+    return a + b;
+  });
+  EXPECT_EQ(f(1, 2), 3);
+  EXPECT_TRUE(called);
+}
+
+TEST(Function, MakeWithCapturingMutableLambda) {
+  bool called = false;
+  auto f = ftl::make_function([&called](int a, int b) mutable {
+    called = true;
+    return a + b;
+  });
+  EXPECT_EQ(f(1, 2), 3);
+  EXPECT_TRUE(called);
+}
+
+TEST(Function, MakeWithThreePointerCapturingLambda) {
+  bool my_bool = false;
+  int my_int = 0;
+  float my_float = 0.f;
+
+  auto f = ftl::make_function(
+      [ptr_bool = &my_bool, ptr_int = &my_int, ptr_float = &my_float](int a, int b) mutable {
+        *ptr_bool = true;
+        *ptr_int = 1;
+        *ptr_float = 1.f;
+
+        return a + b;
+      });
+
+  EXPECT_EQ(f(1, 2), 3);
+
+  EXPECT_TRUE(my_bool);
+  EXPECT_EQ(my_int, 1);
+  EXPECT_EQ(my_float, 1.f);
+}
+
+TEST(Function, MakeWithFreeFunction) {
+  auto f = ftl::make_function<&std::make_unique<int, int>>();
+  std::unique_ptr<int> unique_int = f(1);
+  ASSERT_TRUE(unique_int);
+  EXPECT_EQ(*unique_int, 1);
+}
+
+TEST(Function, CopyToLarger) {
+  int counter = 0;
+  ftl::Function<void()> a{[ptr_counter = &counter] { (*ptr_counter)++; }};
+  ftl::Function<void(), 1> b = a;
+  ftl::Function<void(), 2> c = a;
+
+  EXPECT_EQ(counter, 0);
+  a();
+  EXPECT_EQ(counter, 1);
+  b();
+  EXPECT_EQ(counter, 2);
+  c();
+  EXPECT_EQ(counter, 3);
+
+  b = [ptr_counter = &counter] { (*ptr_counter) += 2; };
+  c = [ptr_counter = &counter] { (*ptr_counter) += 3; };
+
+  b();
+  EXPECT_EQ(counter, 5);
+  c();
+  EXPECT_EQ(counter, 8);
+}
+
+}  // namespace android::test