The availability of methionine is necessary for the normal growth and development of mammals. This essential role derives from the participation of this amino acid, or its derivatives, in several fundamental biologic processes, including protein synthesis; the numerous S-adenosylmethionine (AdoMet)-dependent transmethylation reactions; the formation of the polyamines, spermidine and spermine; the synthesis of cystathionine, cysteine, and other metabolites of the transsulfuration pathway; and the provision of homocysteine, which is necessary for both the metabolism of intracellular folates and the catabolism of choline. During the last two decades, work in several laboratories has defined the potential pathways for methionine metabolism. Concurrently, other studies have established the existence of regulatory processes and have provided some insight into the individual mechanisms which may contribute to this metabolic modulation. In this review, I will summarize the current state of our knowledge.