This report gives a technical overview of air breathing engines for air vehicles, mainly from a performance point of view. The description is focused on engine concepts based on the gas turbine principal, though a section describing the pulse detonation engine (PDE) is also included. The aim of this report is to give an introduction on the subject of air breathing engines. It covers the most important basic principals and measures which are essential to know to carry out performance analyses of engines and air vehicles. The report is introduced by a brief historical review of uses of gas turbine engines for propulsion, followed by a classification of different propulsion concepts based on either the gas turbine principle or the ramjet principle. The elementary theory and the most important performance parameters are outlined, and a brief description of the components included in a gas turbine is given. Furthermore, the performance of gas turbines including simulation tools for estimating the performance and the general design point performance behaviours are outlined. The report ends with an overview of future propulsion concepts.