DEMEOS is a finite‐element based deformable triangular mesh gun code which has been used for almost two decades in the microwave tube field in the design of gridded and diode electron guns, uniform and periodic focusing systems, single and multi‐stage depressed collectors. This paper describes a fully‐relativistic version of the code, RDEMEOS, which has been used in the design of superpower and high impulse klystrons, gyrobeam devices, linacs, and other high power microwave devices. The variable mesh allows beam optical simulations all the way from gun to collector in practical devices. Recently, another version of the basic (nonrelativistic) code has been used to model Spindt cathodes.