Performance data and flow characteristics for subsonic, two-dimensional, straight center line diffusers are presented. The four primary flow regimes which can occur are described and presented as functions of overall diffuser geometry. The performance of both stalled and unstalled diffusers is mapped for a wide range of geometries and inlet boundary layer thicknesses. An understanding of the relationships between flow regime and performance leads to a rational basis for diffuser design. The important maxima of performance and their location on the performance maps are presented. Both the range of data and correlations of optima of performance are extended beyond previous results.