1. It is suggested that the functions of cellular proteinases are far more diverse than has usually been realized. 2. A scheme is proposed in which isolation of a pure proteinase leads to a rather detailed exploration of its biological significance. 3. Representative proteinases from the serine, thiol, carboxyl and metallo-classes are briefly reviewed with emphasis on the relationship between improved understanding at the molecular level and eventual progress in the understanding of some important physiological and pathological processes. 4. An attempt is made to forsee future developments in this area of biochemistry, in terms of both new trends in research, and the eventual application of knowledge of the proteinases to medical problems.