Ion channels are ubiquitous membrane proteins in mammalian cells. Their critical physiological roles include control of the electrical potential across the membrane, facilitation of neuromuscular and neuronal transmission, signal transduction, and regulation of secretion and contractility. The alliance of techniques in biochemistry, electrophysiology, pharmacology, and molecular biology has provided insights into the three-dimensional structure of channel proteins and has allowed specific aspects of functional correlates to be better understood. Promises for the future include molecular genetic approaches to the treatment of disorders such as long QT syndrome and more rational design of drugs targeted to ion channels, including antiarrhythmic agents.