The increasing attention for reducing emissions, which are derived from fuel sources, and the increasing quantity of crude oil with heavy components have resulted in improvement of hydrotreating technology. In order to ensure emission regulations and remove hazardous contents of fuels such as sulfur, nitrogen, olefins, and aromatics, hydrotreating technology has a wide range of applications. Hydrotreatment of middle distillates constitutes 30% of the studies within the patents related to hydrotreating catalysts. The remaining 15% of the corresponding studies are for treatment of lighter distillates. Hydrotreating reactions are exothermic, in many commercial units, quench flows are introduced to cool down the stream and accelerate the desired reactions. Besides nitrogen-containing compounds, metal contents also damage catalyst surface by being deposited on the hydrotreating catalyst. Hydrotreating processes need a new catalyst with increased activity mainly for processing heavy feedstocks and ultradeep desulfurization of diesel fuel.