The two-stroke engine is widely used for competitions 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 within it, and thereby to obtain improved performance.