Catalytic effects of Mo and W compounds in various chemical reactions have been known for decades. Supported catalysts containing Mo or W components as essential active ingredients are the most widely used catalysts in refinery practice. In spite of this, the forms of active Mo or W species and their intimate roles in catalytic reactions are not well understood, Although the presence of sulfides such as MoS2and WS2is frequently reported in these materials, their participation in catalytic reactions is a matter of speculation. In recent years, significant progress has been made in this respect as the result of applying modern techniques, particularly spectroscopy.