Over recent years, our concepts about neurotransmitters have been transformed firstly, because the number of putative transmitters has increased to upwards of 40 candidates and secondly, because our understanding of neurotransmitter function has widened to cover a more diverse range of effects. The aim of this chapter, is not to embrace the medicinal chemistry of all the neurotransmitters and their agonists and antagonists, but more to concentrate on three selected candidates, namely serotonin, histamine and dopamine. Amongst these three examples, there are receptor types which typify both of the major functional classes of neuronal receptor (i.e. ionotropic and metabotropic receptors).