This chapter reviews the operating characteristics of the common types of two-stroke spark-ignition and compression-ignition engines. It discusses the practical engine performance parameters of interest for different types of two-stroke engines. The practical engine performance parameters of interest are power, torque, brake specific fuel consumption, and emissions. Because of its lower pumping and friction losses, the two-stroke crankcase-scavenged engine has inherent theoretical advantages over its four-stroke counterpart in terms of efficiency. Realizing this advantage requires minimizing fuel short-circuiting and improving combustion quality. An important example of a combustion system using air-assist fuel injection technology is the Orbital direct-injected two-stroke engine. The major operating variables that affect two-stroke spark-ignition engine performance, efficiency, and emissions at any given load and speed are spark timing, fuel injection parameters in direct-injection engines, charge composition, exhaust throttling, and ambient conditions.