Embryogenesis of the heart involves the complex cellular differentiation of slow-conducting primary myocardium into the rapidly conducting chamber myocardium of the adult. However, small areas of relatively undifferentiated cells remain to form components of the adult cardiac conduction system (CCS) and nodal tissues. Further investigation has revealed additional areas of nodal-like tissues outside of the established CCS. The embryologic origins of these areas are similar to those of the adult CCS. Under pathologic conditions, these areas can give rise to important clinical arrhythmias. Here, we review the embryologic basis for these proarrhythmic structures within the heart.