Structural studies on several derivatives of molybdopterin, the organic component of the molybdenum cofactor common to virtually all molybdoenzymes, have provided definitive evidence for the presence of a reduced pterin ring with a 6-alkyl side-chain containing an enedithiol and a phosphomonoester in active molybdopterin. The cryptic nature of the cofactor makes structure - function studies on the protein-bound prosthetic group extremely difficult. In this review some of the relevant properties of the pterin ring are presented, and the possible roles of molybdopterin in catalysis are discussed.