To aid the President's Commission on Mental Health as well as the nonspecialist, the uses and developments of psychiatric epidemiology are briefly sketched. In the past few years, methods have been innovated that are capable of making differential diagnoses on a lifetime basis. We need instruments and tools that provide sufficiently detailed information to be of real use for prevention, intervention, and social policy; many such developments are now within our grasp. The problems are prospects of epidemiology, and needed administration and educational supports for future application to the question of who gets ill and why are discussed.