The ion beam formation process and extraction systems are treated in this paper from a rather practical standpoint, with emphasis on, but not strict limitation to, the case of high-current beams extracted from plasma ion sources. Some basic principles of ion extraction optics are discussed and beam quality parameter definitions outlined. Based on the characteristics of an actual extraction system, a general scaling model is then derived for a wide range of beam current and energy values. It allows a determination of the main parameters of systems to be built under specific constraints, or a judgment of the performance of an existing system in terms of one figure of merit. Finally, some examples of special-purpose systems are presented, including multi-aperture, slit, and compound systems. The detailed mechanical contours of these systems are shown to scale in all cases.