Both axial-flow and radial-inflow turbines often employ annular exhaust diffusers to improve the turbine efficiency. The flow typically enters the diffuser in the axial direction for both turbine types. The inlet flow usually has a significant tangential component, particularly when the last stage operates at off-design conditions. The diffuser may also discharge primarily in the axial direction, or it may turn the flow from the axial direction to the radial direction. There are a number of excellent experimental investigations relative to diffuser performance reported in the literature, including two-dimensional, conical and annular diffusers (e.g., Reneau et al. [92], Sovran and Klomp [103], Dolan and Runstadler [104]). These include experimental performance maps that provide useful design guidance but lack the generality required to address the complex flow conditions and various exhaust diffuser configurations encountered in turbines. An approximate diffuser performance analysis model was previously described in chapter 9. That model is based on a vaned diffuser empirical model from Aungier [1, 90]. Although more general than the published performance maps, that model does not account for the influence of the passage shape between the inlet and the discharge. Also, it does not account for the complex interaction of swirling flow on diffuser performance. This chapter describes a more general performance analysis that can address swirling flow in typical exhaust diffuser configurations. Aungier [1, 105] use it to treat the various annular passages encountered in centrifugal compressors, including extensive qualification with experimental data. Aungier [2] generalized it to axial-flow compressor exhaust diffusers and qualified it with data obtained from classical diffuser experiments.