Typical fundamental design choices encountered when designing an engine concept are explained. Examples are separate versus mixed exhaust streams, single- versus multi-stage turbines and dry versus augmented cycles. The commonality of the core components when used in either an aircraft propulsion device or for ground-based power-generation is illustrated and the use of parametric studies to survey a design space and assess the influence of components and their performance characteristics is demonstrated. There is a detailed discussion about turbofans with a reduction gearbox which evaluates the potential benefit as well as the interactions between the components. Introductions are made to efficiency and the mechanics and aerodynamics of a gas turbine engine, along with some of the inevitable compromises. This material is preceded by an outline of basic gas turbine cycles, which evolves from an ideal scenario to realistic cases with losses. Finally, the aircraft mission is considered, as it defines the deliverables and is the foundation of a multitude of design selections. Here, the transition is made from subsonic commercial operation to high-speed, multi-mission requirements and some options available to the designer are described.