This chapter explores the relevant fundamentals and terminology required to understand two-stroke cycle engine combustion. It discusses the two-stroke cycle spark-ignition engine combustion process, and the diesel combustion process. The combustion process is an especially important part of the internal combustion engine's operating cycle. Combustion must release the chemical energy of the fuel—the primary source of energy for the engine—in a relatively short time period between the compression and expansion processes, thereby producing the high-pressure, high-temperature, burned gases that then expand within the cylinder, thereby transferring work to the piston. In a traditional crankcase-scavenged two-stroke engine, the fuel and air are mixed together in the intake system, inducted through the inlet port into the crankcase volume, compressed, and then introduced into the cylinder through the scavenging ports. A different abnormal combustion phenomenon, which often leads to engine destruction, is surface ignition. Surface ignition is ignition of the mixture by a hot spot on the surface of the combustion chamber.