This chapter investigates Hydrodesulfurization (HDS) catalysts and processes in different aspects. HDS is one of the most important industrial operations used to reduce sulfur level of highway and nonroad fuels in the presence of a catalyst. HDS catalysts provide an energetically more favorable pathway, in which sulfur compounds are converted to H2S and hydrocarbon. HDS is widely practiced in petroleum refineries, but it is also used as a postprocess to clean gases produced by coal and biomass gasification. In HDS, the removal of organosulfur compounds is achieved by hydrogenation, which generally results in the formation of saturated hydrocarbon and H2S. In HDS operation, the external mass transfer resistance does not affect the reaction rate significantly, whereas the effect of intraparticle mass transfer resistance on the HDS rate is significant. The inhibition effect of nitrogen-containing organic compounds on HDS activity is dramatic due to their low reactivity.