Nicotinic acetylcholine receptors (nACh receptors2) in jawed vertebrates are prototypical members of the multisubunit, neurotransmitter-gated superfamily of ion channels (ionotropic neurotransmitter receptors; see Section IV for selected seminal papers, reviews, and collections). nACh receptors mediate some of the effects of the endogenous neurotransmitter, acetylcholine, and they are principal biological targets of the tobacco alkaloid, nicotine, which generally (although exceptions exist) mimics acute actions of acetylcholine at nACh receptors. nACh receptors play critical physiological roles throughout the brain and body, mediating excitatory neurotransmission at the vertebrate neuromuscular junction, across autonomic ganglia, and at selected synapses in the brain and spinal cord. Roles have also been suggested for nACh receptors in the modulation of neurotransmitter release as well as in neurotrophism. nACh receptors exist as a variety of types. At least one nACh receptor type is found in developing skeletal muscle, and another is expressed at the mature neuromuscular junction. There is evidence that several nACh receptor types exist in peripheral neurons of autonomic and sensory ganglia and in the brain and spinal cord. Evidence also exists for expression of nACh receptor types in other tissues and cell types, including lymphocytes, fibroblasts, pulmonary neuroendocrine cells, spermatozoa, keratinocytes, granulocytes, chondrocytes, and placenta, as well as in several sensory organs. Classical pharmacological distinctions between some nACh receptor types endure. For example, decamethonium is a selective inhibitor of nACh receptor types in muscle, and hexamethonium is a selective antagonist of nACh receptor types in autonomic ganglia. More contemporary studies are beginning to yield distinctive pharmacological profiles for some nACh receptor types. However, these findings are not yet clearly integrated with molecular definitions of those receptors. This precludes a pharmacological description of nACh receptor types in this report, which instead focuses on matters of nomenclature and on structural bases of nACh receptor diversity.