The purpose of this chapter is to provide an understanding of the design and operating factors responsible for emissions from spark and compression ignition engines. This includes how the air:fuel and equivalence ratio relationships affect motor vehicle emissions; the various changes in engine design that have occurred over the last 40 years in efforts to make engines more efficient while simultaneously reducing emissions for both air pollutants and CO2 emissions; a description of motor vehicle emission control techniques associated with spark ignition engine design and operation; familiarity with major types of exhaust gas control systems and their advantages and limitations; how oxidizing, reducing, and three-way catalysts work and their individual limitations; and familiarity with emission control systems on currently marketed motor vehicles. This chapter also provides a brief description of how compression ignition engines work and what emission control systems are used with them. This chapter now includes sections describing what low-emission/zero-emission vehicles are, the current status of adoption of these vehicles, and the prospects for the future increase in their use.