I on channels are pore-forming proteins that provide pathways for the controlled movement of ions into or out of cells. Ionic movement across cell membranes is critical for essential and physiological processes ranging from control of the strength and duration of the heartbeat to the regulation of insulin secretion in pancreatic β cells. Diseases caused by mutations in genes that encode ion channel subunits or regulatory proteins are referred to as channelopathies. As might be expected based on the diverse roles of ion channels, channelopathies range from inherited cardiac arrhythmias, to muscle disorders, to forms of diabetes. This series of reviews examines the roles of ion channels in health and disease.