Hydroprocessing catalysts based upon the transition metal sulfides have been widely used for over 60 years and catalysts such as Co/Mo/Al2O3remain the industry “workhorses” in hydroprocessing of petroleum-based feedstocks [1]. Such applications include sulfur removal (hydrodesulfurization), nitrogen removal (hydrogenitrogenation), and product quality improvement (hydrotreating, hydroconversion). Original interest (prior to World War II) in these catalysts centered on their activityin the hydrogenation of coal liquids which contain considerable amounts of sulfur, thus maintaining the transition metal in the sulfided state. It was quickly discovered that Co, Ni, Mo, and W sulfides and their mixtures were the most active and least expensive of the transition metal sulfides [2]. Later (post-World War II) their major uses shifted to hydroprocessing of sulfur- and nitrogen-containing petroleum-based feedstocks with Co- and Ni-promoted Mo and W catalysts usually supported on Al2O3. However, as petroleum feedstock supplies dwindle, we are required to process larger quantities of “dirtier” feeds containing larger amounts of sulfur, nitrogen, and metals. In order to meet these requirements in the future, a new generation of transition metal sulfidebased catalysts will be needed which have higher activities, greater selectivity to desired products, and greater resistance to poisons.