This chapter develops the fundamentals and terminology of the gas exchange process in two-stroke cycle engines. It reviews the various scavenging and charging methods used in two-stroke engines, including supercharging, turbocharging, and charge stratification. In two-stroke cycle engines, every outward (or downward) stroke of the piston is a power stroke. In a crankcase-compression engine, the fresh charge is compressed in the crankcase by the underside of the working piston, prior to its admission to the cylinder through the scavenge ducts. The U-cylinder uniflow-scavenged engine is a type of two-stroke cycle engine that has a one-way scavenging path in inverted U-form. This approach, first employed in the Italian Garelli motorcycle, uses two parallel cylinders joined by a common combustion chamber; the exhaust port lies at the bottom of one cylinder and the scavenging ports in the other, while either or both pistons can be used to control the crankcase inlet ports.