CIP systems are now used in almost all sires and types of dairy plant. The history of this development is outlined and the major design influences are set out. The merits of the various types of CIP system in use currently are explained, as are the alternative component options now available. Consideration is given to the engineering principles and other factors that influence the cleaning efficiency of CIP systems. notably the commercial constraints which often determine design decisions. The influence of control equipment on CIP design has been significant and will continue to provide the main springboard for developments in the future.