The heart beat is coordinated by the integrated activities of three myocyte subpopulations: atrial myocytes, ventricular myocytes, and cells of the cardiac conduction system. In this review we discuss the classic fate map and recent retroviral cell lineage studies to better understand the origin, timing, and mechanisms regulating (a) the formation of these three myocyte lineages and (b) the morphogenetic plan underlying formation of the myocardial walls and the conduction system.