The design of a centrifugal compressor stage starts with the selection of achievable design goals and sizing of the stage components to achieve those goals. Candidate stage geometry must be defined in sufficient detail to confirm that it can achieve the design objectives via the aerodynamic performance analysis presented in Chapter 5. An effective preliminary design system will generate the candidate stage design with the minimum input necessary. But it must possess sufficient flexibility to meet the wide range of objectives and constraints a designer may encounter. The preliminary design system should produce a candidate design for the complete stage by optimizing the stage components and matching them to one another. Attempts to accomplish the preliminary design directly to satisfy any arbitrary constraints and performance objectives have been rather ineffective. A performance analysis of the preliminary design usually indicates that the design objectives have not been met and the component geometry produced is usually totally impractical. The direct approach to preliminary design is not well suited to identifying a self-consistent combination of design constraints, performance objectives and key design parameters. Aungier (1995) presents a strategy for preliminary stage design that avoids most of these problems; an expanded description of that procedure is provided here.