ESDU 92042 introduces a Fortran program (available as ESDUpac A9242) for the design or performance assessment of ejectors (or jet pumps) in which the primary and secondary flows may be of the same or of different gases. It uses two approaches to design. The quick design method is based on an analysis of experimental data for single-nozzle designs and, given a minimum of input (selection of entry and required exit pressures, temperatures, mass flow rate and dimensions), will calculate primary nozzle and exit dimensions. The detailed design method, based on one-dimensional flow equations that are detailed, applies also to multi- and annular-nozzle designs. In addition to the input for the quick method, it requires loss factors (in the primary and secondary nozzles, mixing duct and contraction or diffuser) and acceptable maximum design values for four parameters, but will calculate flow conditions throughout the ejector. The performance assessment method for an existing design, also based on the one-dimensional flow equations, calculates conditions throughout the ejector and at exit given the geometry, loss factors and a range of entry conditions. The use of the three methods is illustrated in a worked example. The program is provided as an executable file and within ESDU view.