diff --git a/CMakeLists.txt b/CMakeLists.txt
index b6514c4c4..405343e50 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -11,6 +11,15 @@ PROJECT(fish)
 
 # We are C++11.
 SET(CMAKE_CXX_STANDARD 11)
+SET(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+SET(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -g")
+SET(CMAKE_CXX_FLAGS_RELEASE "-O2")
+SET(DEFAULT_BUILD_TYPE "RelWithDebInfo")
+
+IF(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+    MESSAGE(STATUS "Setting build type to default '${DEFAULT_BUILD_TYPE}'")
+    SET(CMAKE_BUILD_TYPE "${DEFAULT_BUILD_TYPE}")
+ENDIF()
 
 # Disable exception handling.
 ADD_COMPILE_OPTIONS(-fno-exceptions)