Appetite, memory, thermoregulation, sleep, sexual behavior, anxiety, depression, and hallucinogenic behavior are some of the the processes that have been linked with the neurotransmitter serotonin. Whether serotonin plays a primary role or a modulatory role has yet to be determined; nevertheless, it does seem to be involved in numerous actions that would be difficult to explain on the basis of the interaction of a single neurotransmitter with a single type of receptor. However, with the recent discovery of multiple populations of central serotonin binding (receptor?) sites has come a renewed interest in this neurotransmitter, particularly in light of the possibility that its interaction with different types of central sites might explain its various actions. There has been a spillover effect in that there is also increased interest in peripheral serotonergic systems. The entire issue of central serotonin binding sites is relatively new, fraught with controversy, and still in the developmental stage. New binding sites are being reported, multiple-state binding and regulatory processes are being examined, and the functional significance of these sites is being explored. This is probably the most exciting period that serotonin has enjoyed since the pioneering days of serotonin research in the late 1950s and early 1960s. One of the most significant problems facing serotonin research today is a lack of site-selective agonists and antagonists; a continued lack of such took will surely retard further advances in this field. Investigations of functional correlates of central binding, for example, are highly dependent upon the availability of these tools. Following a brief description of the recent advances in the field of serotonin binding sites will be a discussion of the agents that interact with these sites. Hopefully, this will stimulate a search for new site-selective agents. The final section of this Perspective will describe some of the pharmacological effects that these agents produce and the types of functional roles that are currently being considered for these sites.