Several aspects of quenching in fixed-bed hydroprocessing reactors have been reviewed. Examples of processes using hydrogen and liquids as quench fluids are described, and the effects of each quench fluid on the reaction system are discussed. The advantages and disadvantages of each approach on the product quality and process configuration are also highlighted. It was recognized that traditional quench hydrogen helps in improving product quality and reducing catalyst deactivation by coke formation but increases equipment requirements (e.g., compressor) and relative costs, while quench liquid avoids this problem but increases reactor volume.