FOREWORD This chapter is concerned solely with the state of the art in the design of centrifugal or cyclone separators with emphasis on their application in petroleum fluid catalytic cracking units (FCC units). Through the material presented here has general applicability@ it should be noted that many details characteristic of other industrial applications have been omitted. Similarly@ underflow@ particle adhesion@ alternate construction materials@ branching@ multiclones@ common hoppers@ and the like have not been treated in detail. The development of cyclone proportions is traced@ and enough design theories and methods of calculation are covered to allow the reader to make independent designs for any multicomponent particle feed or to evaluate the anticipated performance of an existing installation.