Abstract Ion channels are integral membrane proteins that regulate the traffic of ions in heart cells. They are determinants of the cardiac action potential, which in turn underlies cardiac impulse conduction, excitation-contraction coupling, automaticity, and arrhythmogenesis. Conceptually, the configuration of the cardiac action potential is determined by the sum of ionic current activity at any given time point during the cardiac cycle. Specific cardiac ion channels are discussed along with their clinical relevance according to their participation in the different phases of the cardiac action potential. Various types of cardiac tissues have different ion channel compositions and thus, different configurations of action potentials. Proper cardiac function relies on the coordinated regulation of cardiac rhythm generation and conduction.