ESDU 90025 gives performance maps for straight axis diffusers with sharp transition from an inlet pipe with naturally-developing flow. They apply for an inlet Reynolds number (based on pipe diameter) of one million. They plot static pressure recovery against diffuser area ratio (inlet/outlet) and length ratio (length/inlet radius) and each applies to a specific value of inlet Mach number (from 0.2 to 0.8) and a limited range of inlet pipe length/diameter ratios (from 0 to 35.5). Shown on each map are curves of optimum performance for either given length ratio or given area ratio. The maps, whose use is illustrated by two worked examples, can be applied to determine the performance of a given design or to design an optimum diffuser. An approximate method is suggested for deriving the total head loss from the static pressure recovery. Various other influences on diffuser performance are discussed and illustrated with sketches for specific cases. They include Reynolds number variation, inlet turbulence intensity, inlet velocity profile shape, upstream shock wave/boundary layer interaction, the approach to choking Mach number, fairing the inlet/diffuser junction, and fitting a tailpipe.