All drugs in psychiatry act by influences on neurotransmitter systems. Hence, an appreciation of neurotransmitter disposition helps clarify our understanding of therapeutic actions as well as adverse effects of psychotropic drugs. Moreover, understanding the nuances of neurotransmitters facilitates efforts to develop novel agents. Currently available psychoactive drugs exert their effects through3 or 4 of the longest-known neurotransmitters. Over the past few decades, up to 100 novel neurotransmitter candidates have been identified, each of them as interesting as traditional biogenic amines. It is likely that totally new classes of therapeutic agents will emerge based on these transmitter molecules.