A theory of photoemission is presented which has proven over the last thirty years to provide a very useful description of this process, both for fundamental and practical applications. This is the Three Step Model of Spicer. By treating photoemission in terms of three successive steps-- optical absorption, electrons transport, and escape across the surface, it allows photoemission to be related to parameters of the emitter, such as optical absorption coefficient, electron scattering mechanisms, and the height of the potential barrier at the surface. Using simple equations and established parameters, this theory is very useful in predicting the performance of cathodes and in understanding in detail the unexpected phenomena appearing when photocathodes are pushed into new practical domains.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.