Cyclic nucleotide-regulated cation channels are ion channels whose activation is regulated by the direct binding of cAMP or cGMP to the channel protein. Two structurally related families of channels regulated by cyclic nucleotides have been identified, the cyclic nucleotide-gated channels and the hyperpolarization-activated cyclic nucleotide-gated channels. Cyclic nucleotide-gated channels play a key role in visual and olfactory transduction. Hyperpolarization-activated cyclic nucleotide-gated channels are present in the conduction system of the heart and are involved in the control of cardiac automaticity. Moreover, these channels are widely expressed in central and peripheral neurons, where they control a variety of fundamental processes.