Nicotinic acetylcholine receptors (nAChRs) are major excitatory neurotransmitter receptors in both vertebrates and invertebrates. In insects, nAChRs are the target site for several naturally occurring and synthetic compounds that exhibit potent insecticidal activity. Several compounds isolated from plants are potent agonists or antagonists of nAChRs, suggesting that these may have evolved as a defence mechanism against insects and other herbivores. Nicotine, isolated from the tobacco plant, has insecticidal activity and has been used extensively as a commercial insecticide. Spinosad, a naturally occurring mixture of two macrocyclic lactones isolated from the microorganism Saccharopolyspora spinosa, acts upon nAChRs and has been developed as a commercial insecticide. Since the early 1990s, one of the most widely used and rapidly growing classes of insecticides has been the neonicotinoids. Neonicotinoid insecticides are potent selective agonists of insect nAChRs and are used extensively in both crop protection and animal health applications. As with other classes of insecticides, there is growing evidence for the evolution of resistance to insecticides that act on nAChRs.