During operation, hydroprocessing catalysts undergo deactivation by coke formation, metals deposition, poisoning, etc. In the Literature catalyst deactivation has been extensively studied and reviewed, with focus On the description and understanding of deactivation phenomena. For oil refinery process and catalyst development, the maximum catalyst life, or cycle length, is of prime importance. The catalyst cycle length is determined by initial activities for the relevant reactions, their decline rates, changes in both product yields and qualities during deactivation, and (equipment) fouling by byproducts. The objectives in the development of more stable catalysts are longer cycle lengths in the refineries, intake of heavier feedstocks, and increase of severity of operation. Two processes, hydrocracking and mild hydrocracking, will be highlighted to illustrate how catalyst deactivation is coped with in an industrial environment.