Aircraft require to thrust themselves to accelerate and thus counteract drag forces. In this chapter, we look at the way aircraft engines work. All aircraft propulsion systems are based on the principle of reaction of airflow through a power plant system. The two means for accelerating the airflow surrounding the aircraft that are presented in this chapter are through propellers and jet expansion, which give rise to the so-called propeller engines and jet engines to be studied in Section 6.1 and Section 6.2, respectively. In Section 6.3 the different types of jet engines are studied. A third type of propulsion systems are the rocket engines, but they are used in spacecrafts and lay beyond the scope of this course. An introductory reference on the topic is Newman [3, Chapter 6]. Thorough references are, for instance, Mattingly et al. [2] and Jenkinson et al. [1].