Sphingosine 1-phosphate (SPP) is a phosphorylated derivative of sphingosine, the structural backbone of all sphingolipids, and was initially described as an intermediate in the degradation of long-chain sphingoid bases [1]. However, our discovery that SPP is a potent mitogen for diverse cell types suggested that this metabolite might play other important physiological roles [2]. Over the ensuing decade, SPP has been implicated in a wide variety of biological processes, including mobilization of intracellular calcium, regulation of cytoskeletal organization, as well as cell growth, differentiation, survival, and motility [3]. Attempts to characterize the mechanisms by which SPP exerts such a broad array of actions have revealed that this sphingolipid metabolite is a member of a new class of lipid second messengers that has both intracellular and extracellular actions [4]. Many studies have indicated an intracellular site of action for SPP and shown that it possesses many properties typical of classical second messengers. However, this aspect of SPP function has recently been reviewed elsewhere [3^ 5] and will not be covered here. Instead, this review focuses on the biological functions of SPP that are mediated by a recently discovered family of G protein-coupled receptors.