Gap junctions are specialized regions of adjoining cell membranes composed of numerous intercellular low-resistance channels. In the heart, these channels electrotonically couple adjacent myocytes and synchronize the cardiac action potential. Signaling through gap junction channels may also influence embryogenesis and development. Recent studies have identified the connexin gene family whose protein products assemble to form gap junction channels. Studies of connexin gene expression and function are providing new insights into the behavior of gap junction channels in the heart.