An ion channel directly activated by cGMP was first discovered about ten years ago. Since then, a number of ion channels with the same property (cyclic nucleotide-activated channels) have been reported that are involved in a variety of cell functions. In addition, other channels have been found that are not primarily controlled by cyclic nucleotides but are modulated by them (cyclic nucleotide-modulated channels). These channels likewise have diverse functions and tissue distributions. Both channel classes are reviewed here. Coverage includes the cyclic-nucleotide binding site on these channels, ion permeation, pharmacological blockers, channel gating and modulation, and physiological functions of the channels.