Catalyst deactivation has important consequences for the design of a process and the way it is operated. The nature of the deactivation, and in particular the question whether it can be reversed under conditions which are compatible with the normal operation or whether a separate regeneration treatment of the catalyst is required to restore its activity, as well as the time-scale of the deactivation determine the type of technology that is feasible and process options like reactor type and process configuration. This relationship between deactivation behaviour and process lay-out forms the subject matter of the present paper. The general principles that guide the choices of process type and parameters are illustrated in more detail with examples from the fields of catalytic reforming of petroleum naphtha and hydroprocessing of petroleum residues. In these fields, different catalyst deactivation mechanisms are operative and catalyst deactivation rates can vary widely depending upon feedstock and process parameters. Consequently, different reactor technologies and process configurational choices are possible. The relation between catalyst deactivation behaviour and process design and operation can be viewed from two sides: on the one hand, the deactivation behaviour may dictate the choice between viable process options and may provide an incentive for the development of novel technology that can cope optimally with the demands set by the deactivation of the catalyst. On the other hand, the introduction of novel technological options may widen the scope of a process, e.g. by opening the possibilities to apply novel catalysts or to operate under unconventional conditions that lead to a more economic or otherwise better process, possibilities that were previously barred by catalyst deactivation.