A problem in need of resolution concerns the origin of cardiac progenitor cells and the mechanisms by which these cells are preserved within the cardiac niches. This may be accomplished by migration of progenitor cells from the bone marrow to the myocardium. Alternatively, the progenitor cell compartment in the heart may be maintained by asymmetric division of resident cells. These two possibilities are not mutually exclusive and both exogenous and endogenous progenitor cells may contribute to cardiac homeostasis.