Publisher Summary This chapter provides an overview of coenzymes and prosthetic groups. The terms coenzyme and prosthetic group are frequently used synonymously. Naturally, it is often difficult to decide what is easily dissociable and what is not, for there is a complete range of possibilities. Typically, dissociable coenzymes might better be called co-substrates. They assume the role of a hydrogen or group donor, for example, ATP contributes a phosphate radical in the hexokinase reaction and nicotinamide-adenine dinucleotide accepts hydrogen from the substrate. Many coenzymes are closely related to vitamins. Vitamins are active compounds that must be ingested in food. They are necessary for the proper functioning of life processes and cannot be replaced by other substances. The organism is itself incapable of synthesizing them, except possibly from direct precursors, the pro-vitamins. Almost all coenzymes contain phosphoric acid as an essential component, often in a type of bond called nucleoside phosphate or better nucleotide. Nucleotides consist of a base, a monosaccharide, and phosphoric acid.