diff --git a/CMakeLists.txt b/CMakeLists.txt
index dad564236..ea8dfc6a4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -39,6 +39,39 @@ FetchContent_Declare(
   FIND_PACKAGE_ARGS
 )
 
+# ##############################################################################
+# Initial compilation flags for build types we care about. Setting these in the
+# cache before project() prevents CMake's per-compiler defaults from overwriting
+# them; using DEFINED CACHE{} respects any value the user has already placed in
+# the cache (e.g. via -D on the command line).
+if(NOT DEFINED CACHE{CMAKE_CXX_FLAGS_DEBUG})
+  set(
+    CMAKE_CXX_FLAGS_DEBUG
+    "-Og -g"
+    CACHE STRING
+    "Flags used by the CXX compiler during DEBUG builds."
+  )
+endif()
+
+if(NOT DEFINED CACHE{CMAKE_CXX_FLAGS_RELEASE})
+  set(
+    CMAKE_CXX_FLAGS_RELEASE
+    "-O3 -g0 -DNDEBUG"
+    CACHE STRING
+    "Flags used by the CXX compiler during RELEASE builds."
+  )
+endif()
+
+if(NOT DEFINED CACHE{CMAKE_CXX_FLAGS_RELWITHDEBINFO})
+  set(
+    CMAKE_CXX_FLAGS_RELWITHDEBINFO
+    "-O3 -fno-omit-frame-pointer -g -DNDEBUG"
+    CACHE STRING
+    "Flags used by the CXX compiler during RELWITHDEBINFO builds."
+  )
+endif()
+# ##############################################################################
+
 # Make cetmodules available
 FetchContent_MakeAvailable(cetmodules)
 find_package(cetmodules 4.01.01 REQUIRED)
@@ -49,8 +82,8 @@ project(phlex VERSION 0.1.0 LANGUAGES CXX)
 cet_cmake_env()
 # ##############################################################################
 
-# Set CI/test timeouts to a conservative value to avoid long stalls in CI.
-# Use cache variables so generated CTest/Dart files pick this up when configured.
+# Set CI/test timeouts to a conservative value to avoid long stalls in CI. Use
+# cache variables so generated CTest/Dart files pick this up when configured.
 set(DART_TESTING_TIMEOUT 90 CACHE STRING "Timeout (s) for Dart/CTest runs")
 set(CTEST_TEST_TIMEOUT 90 CACHE STRING "Per-test timeout (s) for CTest")