The return system is probably the least understood component of the centrifugal compressor stage, rivaled only, perhaps, by the volute. The flow through the 180° annular crossover bend, and its interaction with the downstream vane row, presents an extremely complex fluid dynamics problem. The literature offers little guidance relative to design procedures or performance prediction models for this component. Aungier (1993a) is about the only available source offering a detailed aerodynamic design and analysis procedure for the return system. The aerodynamic performance analysis procedure has already been described in Chapter 5. Here, the expanded description of that design procedure is provided. Other than Aungier (1993a), most published experimental and theoretical investigations are attempts to develop a better understanding of these complex flows. Where guidance for a return system design is provided, it usually consists of applying basic engineering judgment to control diffusion rates, passage curvatures and other design parameters, supported by empirical categorization of performance from past development activity. While much remains to be learned about these complex return system flows, the design system of Aungier (1993a) has been used with considerable success. For example, the performance data shown in Figs. 5-13 and 5-19 in Chapter 5 relate to a return system designed with this procedure.