The flow field within the cylinder of internal combustion engines is the most important factor controlling the combustion process. Thus it has a major impact on engine operation. This paper reviews those aspects of gas motion into, within, and out of the engine cylinder that govern the combustion characteristics and breathing capabilities of spark-ignition engines and compression-ignition or diesel engines. Necessary background information on reciprocating engine operating cycles, the primary effect of piston motion and the spark-ignition and diesel engine combustion processes is first summarized. Then the characteristics of flow through inlet and exhaust valves in four-stroke cycle engines, and through ports in the cylinder liner in two-stroke cycle engines are reviewed. These flows govern the airflow through the engine, and set up the in-cylinder flow that controls the subsequent combustion process. The essential features of common in-cylinder flows—the large scale rotating flows set up by the conical intake jet, the creation and development of swirl about the cylinder axis, the flows produced during compression due to combustion chamber shape called squish, flow during the combustion process, and two-stroke scavenging flows—are then described. The turbulence characteristics of these flows are then defined and discussed. Finally, flow phenomena which occur near the walls, which are important to heat transfer and hydrocarbon emissions phenomena, are reviewed. The primary emphasis is on developing insight regarding these important flow phemomena which occur within the cylinder. To this end, results from many different research techniques—experimental and computational, established and new—have been used as resources. It is the rapidly increasing convergence of engine flow information from these many sources that make this an exciting topic with promise of significant practical contributions.