Lineage tracing is a powerful approach for studying the origins and fates of individual cell populations during development. Using an inducible lineage tracing system, Bin Zhou and colleagues (Science 345, 90–94, 2014) have mapped the developmental origins of the coronary vasculature in mice. The authors used tamoxifen-inducible Cre under the control of the Apln promoter to drive expression in coronary vascular endothelial cells. In treating the mice with a single dose of tamoxifen at embryonic day 10.5 and following the fate of the labeled cells postnatally, they found that the labeled cells were restricted to the outer half of the myocardial wall. Conversely, applying single doses of tamoxifen at later time points resulted in progressive labeling of the interventricular septum and the inner myocardial wall. Additional lineage tracing experiments using a promoter expressed selectively in the endocardium showed that these latter waves of vascular endothelial cells emerged directly from the endocardium via lineage conversion during periods of myocardial compaction. The authors propose that this mechanism of lineage conversion allows for more rapid vascular and myocardial growth during the early postnatal period and might provide a new entry point for studying cardiovascular regeneration.