The monograph describes the properties of molybdenum and tungsten and their chemical compounds with other elements, especially oxygen, sulfur, carbon and halides. Data on molybdates and tungstates, on metal alloys of both elements, melts and aqueous solutions of their salts are presented. Special sections are devoted to the qualitative and quantitative analysis of molybdenum and tungsten, methods for their extraction from minerals and ores, and the production of metals.