Perhaps the most difficult practical problem in the study of catalysis is presented by the loss of catalyst activity during reaction. This very complicated phenomenon is made even more confusing by the wide variety of deactivation phenomena and the lack of systematization in this field of research. Consequently little fundamental work has been done on systems in which the catalyst is subject to deactivation. This review proposes to focus on one broad type of catalyst deactivation which also happens to be the most common type of deactivation found in practice.