ESDU 86030 gives step-by-step calculation procedures, based on performance charts, for the optimum design of steam/liquid and steam/gas ejectors. For steam/gas ejectors the data are derived from a wide range of experimental results, but for steam/liquid ejectors there were insufficient data and a theoretical analysis was used. For steam/liquid ejectors a performance prediction method is included that allows off-design behaviour to be estimated. Practical examples using the methods illustrate their application. Mechanical design considerations for the components (primary nozzle, mixing chamber, contraction or diffuser) are described, and the use of annular or multi-nozzle arrangements is considered. Application by multi-staging is discussed. Operating problems in start-up or shut-down are introduced, together with consideration of instability and noise. Limits imposed by the onset of cavitation and vapour binding are outlined for steam/liquid use. ESDU 94046 introduces a program, ESDUpac A9446, that implements the method outlined here.