Molybdenum is crucially important for almost all organisms ranging from ancient single cell microorganisms to modern human beings. The reactions catalyzed by molybdenum and tungsten co-factors involve two electron oxidation or reduction usually accompanied by an oxygen atom transfer from water to substrate or vice versa as part of the carbon, nitrogen and sulfur metabolism. This book deals with the advances of molybdenum and tungsten co-factor model chemistry including a brief survey of the earlier stages of even older inorganic dithiolene chemistry.