Automatic compile-time detection Detect platform characteristics at the time of compilation As implemented in GNU autoconf, though the technique is older Good for users: Fairly easy to use interface Good chance at working on a never-before-seen platform Copes very well with customised platforms Handy place for compile-time customisation (e.g. paths) Not so good for developers: The most popular tool (autoconf) is a little fragile Difficult to reconstruct which options were selected Probably the most popular method