Myocardial K+ currents function to control resting membrane potentials, the heights and durations of action potentials, and refractoriness and automaticity. They are important targets for the actions of transmitters and hormones or drugs known, or postulated, to modulate cardiac functioning. A variety of K+ currents that subserve these functions have now been identified in myocardial cells isolated from different species, as well as in cells isolated from different regions of the heart in the same species. These currents include the voltage-gated K+ types, such as the transient outward (Ito) and delayed rectifier (IK) currents, as well as the inwardly rectifying currents, IK1, IK(ACh), and IK(ATP). The physiological and functional properties of the various K+ currents/channels expressed in different myocardial cell types are the focus of this review. Advances made in cloning K+ channel subunits of the Kv, eag, Kir, and IsK families are discussed, and progress made in identifying the K+ channel subunits expressed in the mammalian myocardium is summarized. The relationships between the various cloned K+ channel subunits and the functional K+ channels characterized electrophysiologically in myocardial cells are explored.