In order to have a sustained flight, a heavier-than-air unmanned aircraft requires a propulsion system.  This chapter is devoted to review the fundamentals of design of a propulsion system. The magnitude of engine power/thrust needed to fulfill a desired aircraft performance is calculated by the technique of matching charts, as discussed in Chap. 3 during the preliminary design phase. In this chapter, other aspects of the propulsion system such as selection of engine type, number of engines, engine selection from manufacturers’ catalogs, sizing propeller (if prop-driven engine), and engine locations will be examined.