diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0d8f7e6..858d32a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,8 +3,11 @@ project(fxos LANGUAGES C CXX)
 find_package(FLEX 2.6)
 
 add_compile_options(
-  -Wall -Wextra -D_GNU_SOURCE -std=c++17 -O2
-  -fmacro-prefix-map=${CMAKE_CURRENT_SOURCE_DIR}/=)
+  -Wall -Wextra -D_GNU_SOURCE -std=c++17
+  -fmacro-prefix-map=${CMAKE_CURRENT_SOURCE_DIR}/=
+  $<$<CONFIG:Debug>:-O0> $<$<CONFIG:Debug>:-g>
+  $<$<CONFIG:Release>:-O2>)
+
 
 # Diagnostic options to clean up output a little bit
 if(FXOS_CLEAN_LOGS)
diff --git a/README.md b/README.md
index e3e5d2a..9767447 100644
--- a/README.md
+++ b/README.md
@@ -33,14 +33,18 @@ requirements.
 * flex (2.6.4)
 * CMake (3.15) and make (eg. 4.2.1)
 
-There are two configure options:
+There are three configure options:
 
+* `-DCMAKE_BUILD_TYPE=Debug|Release`: default is debug.
 * `-DCMAKE_INSTALL_PREFIX=<PATH>`: the install path.  CMake's default is
 `/usr/local`; a viable alternative is `$HOME/.local`.
 * `-DFXOS_CLEAN_LOGS=1`: enables basic compiler output filtering to get error messages under control. Tested with GCC.
 
 ```sh
-% cmake -B build # -DCMAKE_INSTALL_PREFIX=$HOME/.local -DFXOS_CLEAN_LOGS=1
+% cmake -B build
+    # -DCMAKE_BUILD_TYPE=Debug
+    # -DCMAKE_INSTALL_PREFIX=$HOME/.local
+    # -DFXOS_CLEAN_LOGS=1
 % make -C build -j$(nproc) install
 ```