In preliminary design, we run trade studies to make design choices. The figures of merit for optimization should consider the aircraft mission. In this chapter, we show how it is possible at an early stage of the design to generate the flow path, establish the operational viability of the new engine and make a reasonable estimate of its weight. We look at each of the major components or sub-assemblies in a gas turbine and identify the main parameters which influence the flow path and weight. Seeing a diagram of the layout is a good way to check the compatibility and alignment of components. Special attention is placed on disk design as disks are such large contributors to engine weight and operational risks. While it is recognized that absolute numbers generated by these simple methods may not be very accurate, the trends that they identify are useful in making design decisions. Moreover, if more details are available, the results may easily be calibrated against real engines. A by-product of the mechanical design is valuable information such as rotor moment of inertia and heat capacity of casings, which heavily influence transient performance.