A simplified description of hydrocracking is the decomposition of hydrocarbons on the acid sites ofthe catalyst’s support, followed by hydrogenation of the cracking products on the metallic sites. The firststage of this process is analogous to catalytic cracking. Basically, the aim of hydrocracking as well as othercracking processes is the transformation of the heavy fractions of crude oil into light fractions. The use ofthis process is determined by the high quality of some of the products obtained, such as the jet fuel and thelubricating oils of high viscosity index. It is more expensive than catalytic cracking due to the high price ofhydrogen and operation at high pressure. In addition, the role of hydrocracking increased due to the newrequirements for gasoline. Following paper reviews this excellent process used in Petroleum Refineries.Hydrocracking plants are capable of processing a wide variety of feedstocks of different characteristics toproduce a broad range of products. They can be designed and operated to maximize the production of agasoline blending component (called hydrocrackate) or to maximize the production of diesel oil.