Nicotinic acetylcholine receptors (nAChRs) are a diverse family of neurotransmitter-gated ion channels which contain five transmembrane subunits arranged around a central pore. Distinct receptor subtypes are expressed at the vertebrate skeletal neuromuscular junction, in mechanosensory cells and within the central and peripheral nervous systems. A total of 17 nAChR subunits (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon ) have been identified in vertebrate species, which can co-assemble to generate a wide variety of nAChRs. Nicotinic receptors also constitute an abundant and diverse family of receptors in invertebrates. As a consequence of studies which have been conducted with both native and recombinant nAChRs, the subunit composition of nAChRs and the rules governing subunit co-assembly are becoming clearer. In this paper the extent of nAChR subunit diversity and evidence for receptor subunit composition is reviewed.