All Ca(2+) channels are regulated by Ca(2+), a feature that allows them to respond to their own activity and to the activities of neighboring Ca(2+) channels. Inhibition by Ca(2+) protects cells from potentially hazardous increases in cytosolic [Ca(2+)], and stimulation can mediate facilitation and regenerative propagation of Ca(2+) signals. Calmodulin is emerging as a key player in regulation of Ca(2+) channels by Ca(2+), but its role is more complex and more beautiful than might have been imagined.