diff --git a/testing/AbsoluteMouseReport.cpp b/testing/AbsoluteMouseReport.cpp
index 7b9bc3da..33f9e485 100644
--- a/testing/AbsoluteMouseReport.cpp
+++ b/testing/AbsoluteMouseReport.cpp
@@ -16,12 +16,11 @@
 
 #include "testing/AbsoluteMouseReport.h"
 
-#include "Kaleidoscope.h"
-#include "testing/fix-macros.h"
+#include <cstring>  // for memcpy
+#include <vector>   // for vector
 
-#include <cstring>
-
-#include "MouseButtons.h"
+#include "MouseButtons.h"          // for MOUSE_LEFT, MOUSE_MIDDLE, MOUSE_NEXT, MOUSE_PREV, MOUS...
+#include "kaleidoscope/Runtime.h"  // for Runtime, Runtime_
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/AbsoluteMouseReport.h b/testing/AbsoluteMouseReport.h
index 887c2c1b..cc204aa3 100644
--- a/testing/AbsoluteMouseReport.h
+++ b/testing/AbsoluteMouseReport.h
@@ -16,11 +16,11 @@
 
 #pragma once
 
-#include <cstdint>
-#include <vector>
+#include <cstdint>  // for uint16_t, uint32_t, uint8_t, int8_t
+#include <vector>   // for vector
 
-#include "DeviceAPIs/AbsoluteMouseAPI.h"
-#include "HID-Settings.h"
+#include "DeviceAPIs/AbsoluteMouseAPI.h"  // for HID_MouseAbsoluteReport_Data_t
+#include "HID-Settings.h"                 // for HID_REPORTID_MOUSE_ABSOLUTE
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/ConsumerControlReport.cpp b/testing/ConsumerControlReport.cpp
index 8239ae11..8f64f511 100644
--- a/testing/ConsumerControlReport.cpp
+++ b/testing/ConsumerControlReport.cpp
@@ -16,10 +16,10 @@
 
 #include "testing/ConsumerControlReport.h"
 
-#include "Kaleidoscope.h"
-#include "testing/fix-macros.h"
+#include <cstring>  // for memcpy
+#include <vector>   // for vector
 
-#include <cstring>
+#include "kaleidoscope/Runtime.h"  // for Runtime, Runtime_
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/ConsumerControlReport.h b/testing/ConsumerControlReport.h
index 61857f72..93ca97d9 100644
--- a/testing/ConsumerControlReport.h
+++ b/testing/ConsumerControlReport.h
@@ -16,11 +16,11 @@
 
 #pragma once
 
-#include <cstdint>
-#include <vector>
+#include <cstdint>  // for uint32_t, uint16_t, uint8_t
+#include <vector>   // for vector
 
-#include "HID-Settings.h"
-#include "MultiReport/ConsumerControl.h"
+#include "HID-Settings.h"                 // for HID_REPORTID_CONSUMERCONTROL
+#include "MultiReport/ConsumerControl.h"  // for HID_ConsumerControlReport_Data_t
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/ExpectedKeyboardReport.cpp b/testing/ExpectedKeyboardReport.cpp
index bf1f014a..ba4e71b5 100644
--- a/testing/ExpectedKeyboardReport.cpp
+++ b/testing/ExpectedKeyboardReport.cpp
@@ -16,6 +16,9 @@
 
 #include "testing/ExpectedKeyboardReport.h"
 
+#include <cstdint>  // for uint8_t, uint32_t
+#include <set>      // for set
+
 namespace kaleidoscope {
 namespace testing {
 
diff --git a/testing/ExpectedKeyboardReport.h b/testing/ExpectedKeyboardReport.h
index ce967a7b..789dd875 100644
--- a/testing/ExpectedKeyboardReport.h
+++ b/testing/ExpectedKeyboardReport.h
@@ -16,10 +16,10 @@
 
 #pragma once
 
-#include <cstdint>
-#include <vector>
-#include <set>
-#include <string>
+#include <cstdint>  // for uint8_t, uint32_t
+#include <set>      // for set
+
+#include "testing/iostream.h"  // for string
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/ExpectedMouseReport.cpp b/testing/ExpectedMouseReport.cpp
index fb85d7e5..e6f938c8 100644
--- a/testing/ExpectedMouseReport.cpp
+++ b/testing/ExpectedMouseReport.cpp
@@ -16,6 +16,9 @@
 
 #include "testing/ExpectedMouseReport.h"
 
+#include "MultiReport/Mouse.h"    // for (anonymous union)::(anonymous)
+#include "testing/MouseReport.h"  // for MouseReport::ReportData
+
 namespace kaleidoscope {
 namespace testing {
 
diff --git a/testing/ExpectedMouseReport.h b/testing/ExpectedMouseReport.h
index 1d6f2e41..932de2c0 100644
--- a/testing/ExpectedMouseReport.h
+++ b/testing/ExpectedMouseReport.h
@@ -16,12 +16,10 @@
 
 #pragma once
 
-#include <cstdint>
-#include <vector>
-#include <set>
-#include <string>
+#include <cstdint>  // for int8_t, uint32_t, uint8_t
 
-#include "MouseReport.h"
+#include "MouseReport.h"       // for MouseReport, MouseReport::ReportData
+#include "testing/iostream.h"  // for string
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/HIDState.cpp b/testing/HIDState.cpp
index 68c3fb4c..4aac3448 100644
--- a/testing/HIDState.cpp
+++ b/testing/HIDState.cpp
@@ -16,12 +16,14 @@
 
 #include "testing/HIDState.h"
 
-#include "HID-Settings.h"
+// IWYU pragma: no_include <__utility/move.h>
 
-#include "testing/fix-macros.h"
+#include <utility>  // IWYU pragma: keep
+#include <vector>   // for vector
+
+#include "HID-Settings.h"      // for HID_REPORTID_CONSUMERCONTROL, HID_REPORTID_GAMEPAD, HID_RE...
+#include "testing/iostream.h"  // for operator<<, char_traits, cout, ostream, basic_ostream
 
-// TODO(epan): Add proper logging.
-#include <iostream>
 #define LOG(x) std::cout
 
 namespace kaleidoscope {
diff --git a/testing/HIDState.h b/testing/HIDState.h
index 9b93e770..af8449d9 100644
--- a/testing/HIDState.h
+++ b/testing/HIDState.h
@@ -16,15 +16,18 @@
 
 #pragma once
 
-#include "testing/AbsoluteMouseReport.h"
-#include "testing/ConsumerControlReport.h"
-#include "testing/KeyboardReport.h"
-#include "testing/MouseReport.h"
-#include "testing/SystemControlReport.h"
-
-// Out of order due to macro conflicts.
-#include "testing/fix-macros.h"
-#include <memory>
+// IWYU pragma: no_include <__memory/unique_ptr.h>
+
+#include <stddef.h>  // for size_t
+#include <stdint.h>  // for uint8_t
+#include <memory>    // IWYU pragma: keep
+#include <vector>    // for vector
+
+#include "testing/AbsoluteMouseReport.h"    // for AbsoluteMouseReport
+#include "testing/ConsumerControlReport.h"  // for ConsumerControlReport
+#include "testing/KeyboardReport.h"         // for KeyboardReport
+#include "testing/MouseReport.h"            // for MouseReport
+#include "testing/SystemControlReport.h"    // for SystemControlReport
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/KeyboardReport.cpp b/testing/KeyboardReport.cpp
index 89bf822f..429e0f26 100644
--- a/testing/KeyboardReport.cpp
+++ b/testing/KeyboardReport.cpp
@@ -16,10 +16,11 @@
 
 #include "testing/KeyboardReport.h"
 
-#include "Kaleidoscope.h"
-#include "testing/fix-macros.h"
+#include <cstring>  // for memcpy
+#include <vector>   // for vector<>::iterator, vector
 
-#include <cstring>
+#include "kaleidoscope/Runtime.h"   // for Runtime, Runtime_
+#include "kaleidoscope/key_defs.h"  // for HID_KEYBOARD_FIRST_MODIFIER, HID_LAST_KEY
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/KeyboardReport.h b/testing/KeyboardReport.h
index a798d28c..47d06e77 100644
--- a/testing/KeyboardReport.h
+++ b/testing/KeyboardReport.h
@@ -16,11 +16,11 @@
 
 #pragma once
 
-#include <cstdint>
-#include <vector>
+#include <cstdint>  // for uint8_t, uint32_t
+#include <vector>   // for vector
 
-#include "HID-Settings.h"
-#include "MultiReport/Keyboard.h"
+#include "HID-Settings.h"          // for HID_REPORTID_NKRO_KEYBOARD
+#include "MultiReport/Keyboard.h"  // for HID_KeyboardReport_Data_t
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/MouseReport.cpp b/testing/MouseReport.cpp
index 49bb9121..a24b54e0 100644
--- a/testing/MouseReport.cpp
+++ b/testing/MouseReport.cpp
@@ -16,12 +16,9 @@
 
 #include "testing/MouseReport.h"
 
-#include "Kaleidoscope.h"
-#include "testing/fix-macros.h"
+#include <cstring>  // for memcpy
 
-#include <cstring>
-
-#include "MouseButtons.h"
+#include "kaleidoscope/Runtime.h"  // for Runtime, Runtime_
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/MouseReport.h b/testing/MouseReport.h
index 8b4d0258..aa0f9333 100644
--- a/testing/MouseReport.h
+++ b/testing/MouseReport.h
@@ -16,11 +16,11 @@
 
 #pragma once
 
-#include <cstdint>
-#include <vector>
+#include <cstdint>  // for uint8_t, int8_t, uint32_t
 
-#include "HID-Settings.h"
-#include "MultiReport/Mouse.h"
+#include "HID-Settings.h"       // for HID_REPORTID_MOUSE
+#include "MouseButtons.h"       // for MOUSE_LEFT, MOUSE_MIDDLE, MOUSE_NEXT, MOUSE_PREV, MOUSE_R...
+#include "MultiReport/Mouse.h"  // for HID_MouseReport_Data_t
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/SimHarness.cpp b/testing/SimHarness.cpp
index b6acb7e1..7f716d67 100644
--- a/testing/SimHarness.cpp
+++ b/testing/SimHarness.cpp
@@ -16,8 +16,10 @@
 
 #include "testing/SimHarness.h"
 
-#include "testing/fix-macros.h"
-#include <iostream>
+#include <Arduino.h>  // for millis
+
+#include "kaleidoscope/Runtime.h"        // for Runtime, Runtime_
+#include "kaleidoscope/device/device.h"  // for Device, VirtualProps::KeyScanner
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/SimHarness.h b/testing/SimHarness.h
index bda69547..424a43ca 100644
--- a/testing/SimHarness.h
+++ b/testing/SimHarness.h
@@ -16,11 +16,11 @@
 
 #pragma once
 
-#include <cstddef>
-#include <cstdint>
+#include <cstddef>  // for size_t
+#include <cstdint>  // for uint8_t
 
-#include "Kaleidoscope.h"
-#include "testing/fix-macros.h"
+#include "kaleidoscope/KeyAddr.h"  // for KeyAddr
+#include "testing/gtest.h"         // IWYU pragma: keep
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/State.h b/testing/State.h
index b8951b5c..694fd5eb 100644
--- a/testing/State.h
+++ b/testing/State.h
@@ -16,15 +16,11 @@
 
 #pragma once
 
-#include <cstddef>
-#include <cstdint>
-#include <vector>
+// IWYU pragma: no_include <__memory/unique_ptr.h>
 
-#include "testing/HIDState.h"
+#include <memory>  // IWYU pragma: keep
 
-// Out of order due to macro conflicts.
-#include "testing/fix-macros.h"
-#include <memory>
+#include "testing/HIDState.h"  // for HIDState
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/SystemControlReport.cpp b/testing/SystemControlReport.cpp
index 2fb7cc0a..206ee26c 100644
--- a/testing/SystemControlReport.cpp
+++ b/testing/SystemControlReport.cpp
@@ -16,10 +16,9 @@
 
 #include "testing/SystemControlReport.h"
 
-#include "Kaleidoscope.h"
-#include "testing/fix-macros.h"
+#include <cstring>  // for memcpy
 
-#include <cstring>
+#include "kaleidoscope/Runtime.h"  // for Runtime, Runtime_
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/SystemControlReport.h b/testing/SystemControlReport.h
index c5d4edcf..741c565e 100644
--- a/testing/SystemControlReport.h
+++ b/testing/SystemControlReport.h
@@ -16,11 +16,11 @@
 
 #pragma once
 
-#include <cstdint>
-#include <vector>
+#include <cstdint>  // for uint8_t, uint32_t
+#include <vector>   // for vector
 
-#include "HID-Settings.h"
-#include "MultiReport/SystemControl.h"
+#include "HID-Settings.h"               // for HID_REPORTID_SYSTEMCONTROL
+#include "MultiReport/SystemControl.h"  // for HID_SystemControlReport_Data_t
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/VirtualDeviceTest.cpp b/testing/VirtualDeviceTest.cpp
index e7da0619..0bf3282e 100644
--- a/testing/VirtualDeviceTest.cpp
+++ b/testing/VirtualDeviceTest.cpp
@@ -16,10 +16,19 @@
 
 #include "testing/VirtualDeviceTest.h"
 
-#include "HIDReportObserver.h"
-#include "testing/HIDState.h"
-
-#include <bitset>
+// IWYU pragma: no_include <__algorithm/max.h>
+// IWYU pragma: no_include <__tree>
+
+#include <bitset>  // for bitset
+#include <vector>  // for vector
+
+#include "HIDReportObserver.h"       // for HIDReportObserver
+#include "kaleidoscope/Runtime.h"    // for Runtime, Runtime_
+#include "testing/HIDState.h"        // for HIDState, HIDStateBuilder
+#include "testing/KeyboardReport.h"  // for KeyboardReport
+#include "testing/MouseReport.h"     // for MouseReport
+#include "testing/gtest.h"           // for Message, TestPartResult, EXPECT_EQ, ElementsAreArray
+#include "testing/iostream.h"        // for operator<<, basic_ostream, char_traits, string, cerr
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/VirtualDeviceTest.h b/testing/VirtualDeviceTest.h
index 9231c4cd..2c40fdc0 100644
--- a/testing/VirtualDeviceTest.h
+++ b/testing/VirtualDeviceTest.h
@@ -16,18 +16,24 @@
 
 #pragma once
 
-#include <cstddef>
-
-#include "testing/ExpectedKeyboardReport.h"
-#include "testing/ExpectedMouseReport.h"
-#include "testing/SimHarness.h"
-#include "testing/State.h"
-
-// Out of order due to macro conflicts.
-#include "testing/fix-macros.h"
-#include "gmock/gmock.h"
-#include "gtest/gtest.h"
-#include <memory>
+// IWYU pragma: no_include <__memory/unique_ptr.h>
+
+#include <cstddef>           // for size_t
+#include <cstdint>           // for uint32_t, int8_t, uint8_t
+#include <initializer_list>  // for initializer_list
+#include <memory>            // IWYU pragma: keep
+#include <set>               // for set
+#include <vector>            // for vector
+
+#include "kaleidoscope/KeyAddr.h"            // for KeyAddr
+#include "kaleidoscope/key_defs.h"           // for Key
+#include "testing/ExpectedKeyboardReport.h"  // for ExpectedKeyboardReport
+#include "testing/ExpectedMouseReport.h"     // for ExpectedMouseReport
+#include "testing/HIDState.h"                // for HIDState
+#include "testing/SimHarness.h"              // for SimHarness
+#include "testing/State.h"                   // for State
+#include "testing/gtest.h"                   // for Test
+#include "testing/iostream.h"                // for string
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/gtest.h b/testing/gtest.h
new file mode 100644
index 00000000..1931e629
--- /dev/null
+++ b/testing/gtest.h
@@ -0,0 +1,36 @@
+/* -*- mode: c++ -*-
+ * Copyright (C) 2022  Keyboardio, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify it under
+ * the terms of the GNU General Public License as published by the Free Software
+ * Foundation, version 3.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+ * FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+ * details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#undef TEST
+
+// In order to include gtest files, we need to undefine some macros that
+// Arduino.h unwisely exports.  Rahter than including "gtest/gtest.h" (et al)
+// directly, any simulator code should instead include "testing/gtest.h".
+#undef min
+#undef max
+
+// The headers listed here other than "gtest/gtest.h" and "gmock/gmock.h" are
+// only included to prevent IWYU from inserting them directly into simulator
+// source files.  This seems mildly preferable to modifying the gtest files
+// themselves.
+#include "gmock/gmock-matchers.h"   // IWYU pragma: export
+#include "gmock/gmock.h"            // IWYU pragma: export
+#include "gtest/gtest-message.h"    // IWYU pragma: export
+#include "gtest/gtest-test-part.h"  // IWYU pragma: export
+#include "gtest/gtest.h"            // IWYU pragma: export
+#include "gtest/gtest_pred_impl.h"  // IWYU pragma: export
diff --git a/testing/fix-macros.h b/testing/iostream.h
similarity index 65%
rename from testing/fix-macros.h
rename to testing/iostream.h
index a6a63c81..40dca8f7 100644
--- a/testing/fix-macros.h
+++ b/testing/iostream.h
@@ -1,5 +1,5 @@
 /* -*- mode: c++ -*-
- * Copyright (C) 2020  Eric Paniagua (epaniagua@google.com)
+ * Copyright (C) 2022  Keyboardio, Inc.
  *
  * This program is free software: you can redistribute it and/or modify it under
  * the terms of the GNU General Public License as published by the Free Software
@@ -14,12 +14,12 @@
  * this program. If not, see <http://www.gnu.org/licenses/>.
  */
 
-// No `#pragma once` since these undefs need to have every time a Kaleidoscope/
-// Arduino header is included before non-Kaleidoscope/Arduino header. The undefs
-// are needed, due to naming conflicts between Arduino and Googletest.
+#pragma once
 
-#undef min
+// In order to include certain standard library files, we need to undefine some
+// macros that Arduino.h unwisely exports.  Rahter than including <iostream>
+// directly, any simulator code should instead include "testing/iostream.h".
 #undef max
-#undef T
-#undef U
-#undef TEST
+#undef min
+
+#include <iostream>  // IWYU pragma: export
diff --git a/testing/matchers.h b/testing/matchers.h
index bd717568..6de9fe0e 100644
--- a/testing/matchers.h
+++ b/testing/matchers.h
@@ -16,13 +16,12 @@
 
 #pragma once
 
-#include "kaleidoscope/key_defs.h"
-#include "testing/SystemControlReport.h"
+// For some reason, the `MATCHER_P` macro confuses IWYU into trying to include
+// this file in itself, so we need to block it with the following:
+// IWYU pragma: no_include "testing/matchers.h"
 
-// Out of order because `fix-macros.h` clears the preprocessor environment for
-// gtest and gmock.
-#include "testing/fix-macros.h"
-#include "gmock/gmock.h"
+#include "kaleidoscope/key_defs.h"  // for Key
+#include "testing/gtest.h"          // for GMOCK_PP_INTERNAL_FOR_EACH_IMPL_1, GMOCK_PP_INTERNAL_...
 
 namespace kaleidoscope {
 namespace testing {
diff --git a/testing/setup-googletest.h b/testing/setup-googletest.h
index 85cb034e..b7830d89 100644
--- a/testing/setup-googletest.h
+++ b/testing/setup-googletest.h
@@ -18,17 +18,16 @@
 
 #pragma once
 
-#include "kaleidoscope/key_defs/keyboard.h"
-#include "Kaleidoscope.h"
+#include <Kaleidoscope.h>  // IWYU pragma: keep
 
-// Out of order because `fix-macros.h` clears the preprocessor environment for
-// gtest and gmock.
-#include "testing/fix-macros.h"
-#include "gmock/gmock.h"
-#include "gtest/gtest.h"
+// Kaleidoscope.h includes Arduino, which unwisely defines `min` and `max` as
+// preprocessor macros.  We need to undefine these macros before including any
+// files from the standard library.
+#undef min
+#undef max
 
-#include "testing/matchers.h"
-#include "testing/VirtualDeviceTest.h"
+#include "testing/VirtualDeviceTest.h"  // IWYU pragma: keep
+#include "testing/matchers.h"           // IWYU pragma: keep
 
 #define SETUP_GOOGLETEST()                                         \
   void executeTestFunction() {                                     \
diff --git a/tests/issues/951/test/testcase.cpp b/tests/issues/951/test/testcase.cpp
index ffe19b84..e69cb1b3 100644
--- a/tests/issues/951/test/testcase.cpp
+++ b/tests/issues/951/test/testcase.cpp
@@ -15,7 +15,6 @@
  */
 
 #include "Kaleidoscope.h"
-#include "testing/fix-macros.h"
 
 #include "testing/setup-googletest.h"