diff --git a/src/catch2/benchmark/detail/catch_stats.hpp b/src/catch2/benchmark/detail/catch_stats.hpp
index d43925ab..ee1e74d5 100644
--- a/src/catch2/benchmark/detail/catch_stats.hpp
+++ b/src/catch2/benchmark/detail/catch_stats.hpp
@@ -11,17 +11,15 @@
 #ifndef TWOBLUECUBES_CATCH_DETAIL_ANALYSIS_HPP_INCLUDED
 #define TWOBLUECUBES_CATCH_DETAIL_ANALYSIS_HPP_INCLUDED
 
-#include <catch2/benchmark/catch_clock.hpp>
 #include <catch2/benchmark/catch_estimate.hpp>
 #include <catch2/benchmark/catch_outlier_classification.hpp>
 
 #include <algorithm>
-#include <functional>
 #include <vector>
 #include <numeric>
+#include <tuple>
 #include <cmath>
 #include <utility>
-#include <cstddef>
 
 namespace Catch {
     namespace Benchmark {
@@ -64,7 +62,8 @@ namespace Catch {
             template <typename Estimator, typename Iterator>
             sample jackknife(Estimator&& estimator, Iterator first, Iterator last) {
                 auto n = last - first;
-                auto second = std::next(first);
+                auto second = first;
+                ++second;
                 sample results;
                 results.reserve(n);
 
diff --git a/src/catch2/catch_config.cpp b/src/catch2/catch_config.cpp
index a5caeb33..bd311974 100644
--- a/src/catch2/catch_config.cpp
+++ b/src/catch2/catch_config.cpp
@@ -11,6 +11,8 @@
 #include <catch2/internal/catch_stringref.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
 #include <catch2/internal/catch_test_spec_parser.hpp>
+#include <catch2/interfaces/catch_interfaces_tag_alias_registry.hpp>
+
 
 namespace Catch {
 
diff --git a/src/catch2/catch_session.cpp b/src/catch2/catch_session.cpp
index 3abd8410..80767184 100644
--- a/src/catch2/catch_session.cpp
+++ b/src/catch2/catch_session.cpp
@@ -18,14 +18,11 @@
 #include <catch2/internal/catch_random_number_generator.hpp>
 #include <catch2/internal/catch_startup_exception_registry.hpp>
 #include <catch2/internal/catch_text.hpp>
-#include <catch2/internal/catch_stream.hpp>
 #include <catch2/internal/catch_windows_h_proxy.hpp>
 #include <catch2/reporters/catch_reporter_listening.hpp>
 
-#include <cstdlib>
 #include <iomanip>
 #include <set>
-#include <iterator>
 
 namespace Catch {
 
diff --git a/src/catch2/catch_session.hpp b/src/catch2/catch_session.hpp
index 52bf9f34..8d646382 100644
--- a/src/catch2/catch_session.hpp
+++ b/src/catch2/catch_session.hpp
@@ -10,7 +10,6 @@
 
 #include <catch2/internal/catch_commandline.hpp>
 #include <catch2/catch_config.hpp>
-#include <catch2/internal/catch_text.hpp>
 #include <catch2/internal/catch_unique_ptr.hpp>
 
 namespace Catch {
diff --git a/src/catch2/catch_test_case_info.cpp b/src/catch2/catch_test_case_info.cpp
index a4bcb11c..e63721ae 100644
--- a/src/catch2/catch_test_case_info.cpp
+++ b/src/catch2/catch_test_case_info.cpp
@@ -12,10 +12,9 @@
 #include <catch2/interfaces/catch_interfaces_testcase.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
 
+#include <cassert>
 #include <cctype>
-#include <exception>
 #include <algorithm>
-#include <sstream>
 
 namespace Catch {
 
diff --git a/src/catch2/catch_test_spec.cpp b/src/catch2/catch_test_spec.cpp
index 88966ec1..6c408934 100644
--- a/src/catch2/catch_test_spec.cpp
+++ b/src/catch2/catch_test_spec.cpp
@@ -7,7 +7,6 @@
 
 #include <catch2/catch_test_spec.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
-#include <catch2/interfaces/catch_interfaces_config.hpp>
 #include <catch2/catch_test_case_info.hpp>
 
 #include <algorithm>
diff --git a/src/catch2/internal/catch_assertion_handler.cpp b/src/catch2/internal/catch_assertion_handler.cpp
index 54153462..b666ba31 100644
--- a/src/catch2/internal/catch_assertion_handler.cpp
+++ b/src/catch2/internal/catch_assertion_handler.cpp
@@ -7,8 +7,6 @@
  */
 
 #include <catch2/internal/catch_assertion_handler.hpp>
-#include <catch2/catch_assertion_result.hpp>
-#include <catch2/interfaces/catch_interfaces_runner.hpp>
 #include <catch2/interfaces/catch_interfaces_config.hpp>
 #include <catch2/internal/catch_context.hpp>
 #include <catch2/internal/catch_enforce.hpp>
diff --git a/src/catch2/internal/catch_clara.hpp b/src/catch2/internal/catch_clara.hpp
index 633db558..2420a1d8 100644
--- a/src/catch2/internal/catch_clara.hpp
+++ b/src/catch2/internal/catch_clara.hpp
@@ -9,6 +9,9 @@
 #ifndef TWOBLUECUBES_CATCH_CLARA_H_INCLUDED
 #define TWOBLUECUBES_CATCH_CLARA_H_INCLUDED
 
+// For CATCH_CONFIG_CONSOLE_WIDTH. We should refactor this out somewhere else.
+#include <catch2/catch_config.hpp>
+
 // Use Catch's value for console width (store Clara's off to the side, if present)
 #ifdef CLARA_CONFIG_CONSOLE_WIDTH
 #define CATCH_TEMP_CLARA_CONFIG_CONSOLE_WIDTH CATCH_CLARA_TEXTFLOW_CONFIG_CONSOLE_WIDTH
diff --git a/src/catch2/internal/catch_commandline.cpp b/src/catch2/internal/catch_commandline.cpp
index 2759eb8e..0f24fc95 100644
--- a/src/catch2/internal/catch_commandline.cpp
+++ b/src/catch2/internal/catch_commandline.cpp
@@ -8,11 +8,12 @@
 
 #include <catch2/internal/catch_commandline.hpp>
 
+#include <catch2/catch_config.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
-
 #include <catch2/interfaces/catch_interfaces_registry_hub.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter.hpp>
 
+#include <algorithm>
 #include <fstream>
 #include <ctime>
 
diff --git a/src/catch2/internal/catch_commandline.hpp b/src/catch2/internal/catch_commandline.hpp
index 0824397d..e0bb9b4d 100644
--- a/src/catch2/internal/catch_commandline.hpp
+++ b/src/catch2/internal/catch_commandline.hpp
@@ -8,11 +8,12 @@
 #ifndef TWOBLUECUBES_CATCH_COMMANDLINE_HPP_INCLUDED
 #define TWOBLUECUBES_CATCH_COMMANDLINE_HPP_INCLUDED
 
-#include <catch2/catch_config.hpp>
 #include <catch2/internal/catch_clara.hpp>
 
 namespace Catch {
 
+    struct ConfigData;
+
     clara::Parser makeCommandLineParser( ConfigData& config );
 
 } // end namespace Catch
diff --git a/src/catch2/internal/catch_common.cpp b/src/catch2/internal/catch_common.cpp
index aaf08079..eec0ed6f 100644
--- a/src/catch2/internal/catch_common.cpp
+++ b/src/catch2/internal/catch_common.cpp
@@ -7,8 +7,6 @@
  */
 
 #include <catch2/internal/catch_common.hpp>
-#include <catch2/internal/catch_context.hpp>
-#include <catch2/interfaces/catch_interfaces_config.hpp>
 
 #include <cstring>
 #include <ostream>
diff --git a/src/catch2/internal/catch_common.hpp b/src/catch2/internal/catch_common.hpp
index a46c46b3..45a1eca5 100644
--- a/src/catch2/internal/catch_common.hpp
+++ b/src/catch2/internal/catch_common.hpp
@@ -20,8 +20,6 @@
 #endif
 
 #include <iosfwd>
-#include <string>
-#include <cstdint>
 
 // We need a dummy global operator<< so we can bring it into Catch namespace later
 struct Catch_global_namespace_dummy {};
diff --git a/src/catch2/internal/catch_console_colour.cpp b/src/catch2/internal/catch_console_colour.cpp
index 6516018b..a7bce470 100644
--- a/src/catch2/internal/catch_console_colour.cpp
+++ b/src/catch2/internal/catch_console_colour.cpp
@@ -23,7 +23,7 @@
 #include <catch2/internal/catch_debugger.hpp>
 #include <catch2/internal/catch_windows_h_proxy.hpp>
 
-#include <sstream>
+#include <ostream>
 
 namespace Catch {
     namespace {
diff --git a/src/catch2/internal/catch_context.hpp b/src/catch2/internal/catch_context.hpp
index b52a2267..9f446f24 100644
--- a/src/catch2/internal/catch_context.hpp
+++ b/src/catch2/internal/catch_context.hpp
@@ -13,7 +13,6 @@ namespace Catch {
     struct IResultCapture;
     struct IRunner;
     struct IConfig;
-    struct IMutableContext;
 
     struct IContext
     {
diff --git a/src/catch2/internal/catch_enum_values_registry.cpp b/src/catch2/internal/catch_enum_values_registry.cpp
index 7e7deb87..4e410f93 100644
--- a/src/catch2/internal/catch_enum_values_registry.cpp
+++ b/src/catch2/internal/catch_enum_values_registry.cpp
@@ -9,7 +9,6 @@
 #include <catch2/internal/catch_string_manip.hpp>
 #include <catch2/internal/catch_stream.hpp>
 
-#include <map>
 #include <cassert>
 
 namespace Catch {
diff --git a/src/catch2/internal/catch_enum_values_registry.hpp b/src/catch2/internal/catch_enum_values_registry.hpp
index 080af01e..3100136b 100644
--- a/src/catch2/internal/catch_enum_values_registry.hpp
+++ b/src/catch2/internal/catch_enum_values_registry.hpp
@@ -10,6 +10,7 @@
 
 #include <catch2/interfaces/catch_interfaces_enum_values_registry.hpp>
 #include <catch2/internal/catch_unique_ptr.hpp>
+#include <catch2/internal/catch_stringref.hpp>
 
 #include <vector>
 
diff --git a/src/catch2/internal/catch_list.cpp b/src/catch2/internal/catch_list.cpp
index 47590ee5..ab3db4df 100644
--- a/src/catch2/internal/catch_list.cpp
+++ b/src/catch2/internal/catch_list.cpp
@@ -13,18 +13,12 @@
 #include <catch2/interfaces/catch_interfaces_testcase.hpp>
 
 #include <catch2/internal/catch_context.hpp>
+#include <catch2/catch_config.hpp>
 #include <catch2/internal/catch_stream.hpp>
-
-#include <catch2/internal/catch_console_colour.hpp>
 #include <catch2/internal/catch_test_spec_parser.hpp>
-#include <catch2/catch_tostring.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
 #include <catch2/catch_test_case_info.hpp>
 
-#include <limits>
-#include <algorithm>
-#include <set>
-
 namespace Catch {
     namespace {
 
diff --git a/src/catch2/internal/catch_list.hpp b/src/catch2/internal/catch_list.hpp
index 638f05b0..0c47d875 100644
--- a/src/catch2/internal/catch_list.hpp
+++ b/src/catch2/internal/catch_list.hpp
@@ -8,7 +8,6 @@
 #ifndef TWOBLUECUBES_CATCH_LIST_H_INCLUDED
 #define TWOBLUECUBES_CATCH_LIST_H_INCLUDED
 
-#include <catch2/catch_config.hpp>
 #include <catch2/internal/catch_stringref.hpp>
 
 #include <set>
@@ -18,6 +17,8 @@
 namespace Catch {
 
     struct IStreamingReporter;
+    class Config;
+
 
     struct ReporterDescription {
         std::string name, description;
diff --git a/src/catch2/internal/catch_run_context.cpp b/src/catch2/internal/catch_run_context.cpp
index b85f9ea0..72b7b63d 100644
--- a/src/catch2/internal/catch_run_context.cpp
+++ b/src/catch2/internal/catch_run_context.cpp
@@ -1,4 +1,5 @@
 #include <catch2/interfaces/catch_interfaces_generatortracker.hpp>
+#include <catch2/interfaces/catch_interfaces_config.hpp>
 #include <catch2/internal/catch_run_context.hpp>
 #include <catch2/internal/catch_compiler_capabilities.hpp>
 #include <catch2/internal/catch_context.hpp>
@@ -8,6 +9,7 @@
 #include <catch2/internal/catch_stream.hpp>
 #include <catch2/catch_timer.hpp>
 #include <catch2/internal/catch_output_redirect.hpp>
+#include <catch2/internal/catch_assertion_handler.hpp>
 
 #include <cassert>
 #include <algorithm>
diff --git a/src/catch2/internal/catch_run_context.hpp b/src/catch2/internal/catch_run_context.hpp
index d0664e6e..f7d3660b 100644
--- a/src/catch2/internal/catch_run_context.hpp
+++ b/src/catch2/internal/catch_run_context.hpp
@@ -10,14 +10,13 @@
 
 #include <catch2/interfaces/catch_interfaces_runner.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter.hpp>
-#include <catch2/catch_config.hpp>
 #include <catch2/internal/catch_test_registry.hpp>
 #include <catch2/catch_test_case_info.hpp>
-#include <catch2/internal/catch_test_macro_impl.hpp>
 #include <catch2/catch_message.hpp>
 #include <catch2/catch_totals.hpp>
 #include <catch2/internal/catch_test_case_tracker.hpp>
-#include <catch2/internal/catch_assertion_handler.hpp>
+#include <catch2/catch_assertion_info.hpp>
+#include <catch2/catch_assertion_result.hpp>
 
 #include <string>
 
@@ -25,6 +24,7 @@ namespace Catch {
 
     struct IMutableContext;
     struct IGeneratorTracker;
+    struct IConfig;
 
     ///////////////////////////////////////////////////////////////////////////
 
diff --git a/src/catch2/internal/catch_tag_alias_registry.cpp b/src/catch2/internal/catch_tag_alias_registry.cpp
index 7a3d397e..b1241dd1 100644
--- a/src/catch2/internal/catch_tag_alias_registry.cpp
+++ b/src/catch2/internal/catch_tag_alias_registry.cpp
@@ -12,8 +12,6 @@
 #include <catch2/interfaces/catch_interfaces_registry_hub.hpp>
 #include <catch2/internal/catch_string_manip.hpp>
 
-#include <sstream>
-
 namespace Catch {
 
     TagAliasRegistry::~TagAliasRegistry() {}
diff --git a/src/catch2/internal/catch_tag_alias_registry.hpp b/src/catch2/internal/catch_tag_alias_registry.hpp
index 47cc726b..c76c8c1c 100644
--- a/src/catch2/internal/catch_tag_alias_registry.hpp
+++ b/src/catch2/internal/catch_tag_alias_registry.hpp
@@ -12,6 +12,7 @@
 #include <catch2/catch_tag_alias.hpp>
 
 #include <map>
+#include <string>
 
 namespace Catch {
 
diff --git a/src/catch2/internal/catch_test_case_registry_impl.cpp b/src/catch2/internal/catch_test_case_registry_impl.cpp
index b7ebe977..3aca929c 100644
--- a/src/catch2/internal/catch_test_case_registry_impl.cpp
+++ b/src/catch2/internal/catch_test_case_registry_impl.cpp
@@ -17,9 +17,7 @@
 #include <catch2/catch_test_spec.hpp>
 
 #include <algorithm>
-#include <iterator>
-#include <random>
-#include <sstream>
+#include <set>
 
 namespace Catch {
 
diff --git a/src/catch2/internal/catch_test_case_registry_impl.hpp b/src/catch2/internal/catch_test_case_registry_impl.hpp
index 01a2c349..4fd692fe 100644
--- a/src/catch2/internal/catch_test_case_registry_impl.hpp
+++ b/src/catch2/internal/catch_test_case_registry_impl.hpp
@@ -12,8 +12,6 @@
 #include <catch2/interfaces/catch_interfaces_config.hpp>
 
 #include <vector>
-#include <set>
-#include <algorithm>
 
 namespace Catch {
 
diff --git a/src/catch2/internal/catch_test_case_tracker.cpp b/src/catch2/internal/catch_test_case_tracker.cpp
index 4341145d..6636820a 100644
--- a/src/catch2/internal/catch_test_case_tracker.cpp
+++ b/src/catch2/internal/catch_test_case_tracker.cpp
@@ -13,7 +13,6 @@
 #include <algorithm>
 #include <cassert>
 #include <memory>
-#include <sstream>
 
 #if defined(__clang__)
 #    pragma clang diagnostic push
diff --git a/src/catch2/internal/catch_test_spec_parser.cpp b/src/catch2/internal/catch_test_spec_parser.cpp
index d82cb5e0..38f3f136 100644
--- a/src/catch2/internal/catch_test_spec_parser.cpp
+++ b/src/catch2/internal/catch_test_spec_parser.cpp
@@ -8,6 +8,7 @@
 #include <catch2/internal/catch_test_spec_parser.hpp>
 
 #include <catch2/internal/catch_string_manip.hpp>
+#include <catch2/interfaces/catch_interfaces_tag_alias_registry.hpp>
 
 
 namespace Catch {
diff --git a/src/catch2/internal/catch_test_spec_parser.hpp b/src/catch2/internal/catch_test_spec_parser.hpp
index 087a8737..c070ffc1 100644
--- a/src/catch2/internal/catch_test_spec_parser.hpp
+++ b/src/catch2/internal/catch_test_spec_parser.hpp
@@ -14,10 +14,14 @@
 #endif
 
 #include <catch2/catch_test_spec.hpp>
-#include <catch2/interfaces/catch_interfaces_tag_alias_registry.hpp>
+
+#include <vector>
+#include <string>
 
 namespace Catch {
 
+    struct ITagAliasRegistry;
+
     class TestSpecParser {
         enum Mode{ None, Name, QuotedName, Tag, EscapedName };
         Mode m_mode = None;
diff --git a/src/catch2/matchers/catch_matchers_floating.cpp b/src/catch2/matchers/catch_matchers_floating.cpp
index f574053f..1f79dfe5 100644
--- a/src/catch2/matchers/catch_matchers_floating.cpp
+++ b/src/catch2/matchers/catch_matchers_floating.cpp
@@ -17,7 +17,6 @@
 #include <cstdint>
 #include <cstring>
 #include <sstream>
-#include <type_traits>
 #include <iomanip>
 #include <limits>
 
diff --git a/src/catch2/matchers/catch_matchers_templated.hpp b/src/catch2/matchers/catch_matchers_templated.hpp
index 89c85fcc..c6476595 100644
--- a/src/catch2/matchers/catch_matchers_templated.hpp
+++ b/src/catch2/matchers/catch_matchers_templated.hpp
@@ -10,7 +10,6 @@
 
 #include <array>
 #include <algorithm>
-#include <memory>
 #include <string>
 #include <type_traits>
 #include <utility>
diff --git a/src/catch2/reporters/catch_reporter_listening.cpp b/src/catch2/reporters/catch_reporter_listening.cpp
index 860ee82b..b4c91205 100644
--- a/src/catch2/reporters/catch_reporter_listening.cpp
+++ b/src/catch2/reporters/catch_reporter_listening.cpp
@@ -6,7 +6,8 @@
  *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  */
 
- #include <catch2/reporters/catch_reporter_listening.hpp>
+#include <catch2/reporters/catch_reporter_listening.hpp>
+
 #include <cassert>
 
 namespace Catch {
diff --git a/src/catch2/reporters/catch_reporter_teamcity.cpp b/src/catch2/reporters/catch_reporter_teamcity.cpp
index 2cb51bed..5634971d 100644
--- a/src/catch2/reporters/catch_reporter_teamcity.cpp
+++ b/src/catch2/reporters/catch_reporter_teamcity.cpp
@@ -10,6 +10,8 @@
 #include <catch2/internal/catch_text.hpp>
 #include <catch2/catch_test_case_info.hpp>
 
+#include <cassert>
+
 namespace Catch {
 
     namespace {