Potassium channels are integral membrane proteins that selectively transport K+ across the cell membrane. They are present in all mammalian cells and have a wide variety of roles in both excitable and nonexcitable cells. The phenotypic diversity required to accomplish their various roles is created by differences in conductance, the timecourse and mechanisms of different gating events, and the interaction of channels with a variety of accessory proteins. Through the integration of biophysical, molecular, structural, and theoretical studies, significant progress has been made toward understanding the structural basis of K+ channel function, and diseases associated with K+ channel dysfunction.