The selection of an appropriate reactor for hydroprocessing catalysis research primarily depends on the type of information to be generated, the reliability and reproducibility of the information required, and the cost involved to generate such information. In this article, the various reactors and approaches that. have been used at different stages of hydroprocessing catalysis research and process development are reviewed. With the development of the dilution technique, substantial progress has taken place in reducing the size of reactor used for hydroprocessing process development studies. This has decreased the cost of catalyst evaluation as well as increased the operating safety. However, the selection of an appropriate size of diluent is very important. Studies comparing the differences in the behavior between up-flow and down-flow modes of operation of a fixed bed reactor are analyzed. An attempt has been made to provide guidelines for selecting the appropriate reactor and methodology to evaluate hydroprocessing catalysts. Reactors that are commonly used for the kinetic studies of hydroprocessing reactions are also reviewed in this article.