The origin and function of normal monocytes and macrophages have been clearly defined by extensive investigations in human and in animal models. The central importance ofthis cell system for the biological defense mecha- nisms is well established: phagocytosis, inactivation and destruction of organic and inorganic materials, an important role in the initiation ofhumo- ral and cell mediated immunological responses, and the secretion of a varie- ty of chemical mediator and effector substances are the most important fea- tures of this ontogenetically ancient cell system. However, the data on this cellular system are rather recent, and this may explain why relatively little attention has been payed to the pathology of the monocyte-macrophage system (MMS) until now. In addition, this monograph should focus attention on the secondarypa- thophysiological implications of the MMS in disorders not primarily origi- nating from this system. Several techniques are available to identify even abnormal individuals of this cell system and, therefore, can be employed for the study of severely altered or neoplastic monocytic cells.