Ionic currents over the plasma membrane through channels are the cornerstone of excitable cells. Human cardiomyocytes are excitable and continuously cycle between a depolarized and a repolarized state every second throughout human life, initiating and coordinating cardiac pump function. Ion channels selective for potassium (K+) critically participate in cellular repolarization and contribute to stabilizing the diastolic membrane potential, thus shaping the cardiac action potential. Four different subfamilies of potassium channels are present in the heart: small conductance, calcium-activated potassium channels (SK or K-Ca(2)), inwardly rectifying potassium channels (K-ir), two-pore-domain potassium channels (K-2P), and voltage-gated potassium channels (K-V). In the present review, the structure and biophysical function of these cardiac potassium ion channels are reviewed. Moreover, rectification, inactivation, and current dependency on the extracellular potassium concentration are explained.