Electrical motors and generators used on mobile platforms--including aircraft and ship and land vehicles--are required to exhibit as high a power density as practically possible. With the advent of high-energy permanent magnets, it appears possible to build permanent magnet machines which can compete with electrically excited machines in terms of power density. This paper presents a discussion of the power density limitations as they apply to synchronous-type permanent magnet machines using rare earth cobalt permanent magnets. It also presents guidelines for future material development necessary to improve the power density of this class of machines.