Molybdenum is an essential trace element for virtually all life forms. It functions as a co-factor for a number of enzymes that catalyse important chemical transformations in the global carbon, nitrogen, and sulphur cycles. Thus, molybdenum-dependent enzymes are not only required for human health, but also for the health of our ecosystem. Molybdenum and its compounds are also very important in catalysis and in medicine, so it is not surprising that its biological and coordination chemistry remain very active areas of research. The enormous chemistry of molybdenum, much of which remains untapped, was investigated over many years by two icons: Edward I Stiefel and A Geoffrey Sykes. Ed and Geoff, as they were affectionately called by many of their friends and research students, are being honoured for their contributions to research on molybdenum with this textbook.