Internal combustion engines, forming part of a degree or similar course in thermodynamics, are treated in the order: ideal cycles, cycles using actual working substances in ideal engines, consideration in detail of spark-ignition and compression-ignition engines, petrol and oil engines being taken as the representative types. The progressive or historical development of the subject is emphasized and considerable reference is made to laboratory work. Lastly, comments are offered on post-graduate work. For the petrol engine, the combustion processes are studied at length, based on the work of Ricardo, Tizard, and Pye, which led to an understanding of detonation and to the octane number as a criterion of fuel quality. For the compression-ignition engine, the combustion processes are similarly studied, the basic work of Le Mesurier and Stansfield and of Boerlage and Broeze, leading to the cetane number, receiving particular attention. Several kinds of comparison are drawn between the characteristics of spark-ignition and compression-ignition engines. For both types of engine, the combustion processes are considered in relation to the performance of engines in the laboratory. The selection of material in view of the restricted time of the course is touched upon. In appendixes will be found recommended laboratory equipment and suggested works of reference of value to an instructor.