Cyclic nucleotide-gated (CNG) channels play important roles in both visual (Yau & Baylor 1989) and olfactory (Zufall et al 1994) signal transduction. The cloning of the gene coding for a rod photoreceptor channel (Kaupp et al 1989) and the subsequent cloning of related genes from olfactory neurons (Dhallan et al 1990, Ludwig et al 1990, Goulding et al 1992) has sparked much progress over the past several years in elucidating the structural bases for the function of the CNG channels. One of the surprising features to emerge from these cloning studies was that the CNG channels are structurally homologous to the voltage-gated channels (Jan & Jan 1990) despite the fact that the CNG channels are gated by the binding of a ligand-cAMP or cGMP-and not by voltage. In this review we focus on recent studies of the relationship between the structure and function of the CNG channels. Given the homology between CNG channels and voltage-gated channels, such studies are likely to provide important general information about the structure and function of a wide variety of channel types.