This volume describes preparation and properties of the enormous number of molybdenum oxide sulfide ions. The monomeric anions display an impressive range of colors - yellow to a deep reddish orange - which in 1826 fascinated Berzelius. Nowadays, however, the stimulus for research and development springs from bioinorganic chemistry. Numerous molybdenum chalcogenide halides are known: the most investigated have been those containing Mo6 clusters. In addition, the volume contains the binary molybdenum selenides; of these, the most important is the heavily studied MoSe2. Among the molybdenum dichalcogenides, MoSe2 shows the most promising photoelectrochemical properties, making it attractive for use in electrochemical solar cells. All binary molybdenum selenides reversibly intercalate a variety of metals - of importance for use in rechargeable batteries.