The two-stroke engine is widely used in both motorcycle racing and kart racing, and in very large numbers in model car, boat and aircraft competition. The mechanical simplicity of the two-stroke engine gives it tremendous appeal, and makes it a tempting target for tuning operations, but the key to successful design, development and modification is knowledge of the engine¦s operating principles. This in-depth technical study of two-stroke theory and practice is intended to help would-be engine tuners to better understand the engine and the processes taking place within it, and thereby to obtain improved performance.