Engineering design is how we turn ideas into hardware and where engine models are used to capture a design concept and demonstrate how well it performs before the hardware is produced. Preliminary design is where many decisions are made in a new engine program and where the results will determine how significant corporate resources will be committed for many years. In this chapter, we look at the preliminary design of complete engine systems and consider how they fit into the overall project in terms of the mission requirements, the cycle, component design and development and on to manufacture and testing. We propose a workflow starting from the core compressor and then working aft and forward until all components are addressed. We describe the relationships between components, the implementation of design constraints into the overall model and the use of trade studies to make the major design decisions. We illustrate the role played by performance beyond preliminary design not only in supplying the specialist component designers with the information they require but also in validating the cycle deliverables during the development phases of a program.