The heart is one of the first organs to start and complete its differentiation during embryonic development. However, relatively little is known about the molecular, cellular and functional fingerprint of early embryonic heart cells. To unravel this, transgenic embryonic stem cell lines and their in vitro differentiation into the cardiac lineage as well as transgenic mouse models have been used. This approach enabled the identification and functional characterization of early embryonic pacemaker-, atrial- and ventricular cardiomyocytes. Furthermore, these studies showed that pacemaking in the early embryonic heart is generated by intracellular Ca2+ oscillations rather than membrane delimited events. Long term, we hope that mechanistic insight into the build up of embryonic heart function will provide a better pathophysiological understanding of the diseased adult heart.