Stem cells are capable of indefinite self-renewal and can give rise to all tissues derived from the three primary germ layers. Embryonic stem cells and, to a lesser extent, bone marrow–derived cells, share these properties, while skeletal myoblasts should be considered as precursor cells for myotubes, because of their restricted commitment to this specific skeletal muscle lineage. Evidence for the differentiation of bone marrow– derived stem cells into cardiomyocytes is inconclusive, and available data indicate that the major effect of the transplantation of these cells is to increase angiogenesis, probably through secretion of angiogenic growth factors rather than conversion into an endothelial- like phenotype. To date, clinical trials have documented the feasibility and safety of transplantation of bone marrow cells and skeletal myoblasts, with the caveat of a proarrhythmic risk associated with the latter. However, efficacy data can only be generated by randomized trials, some of which are already under way. Initial findings suggest that bone marrow cells may be particularly suitable for increasing angiogenesis in patients with acute myocardial infarction or intractable ischemia, whereas myoblasts may be more effective in improving function in the setting of advanced ischemic heart failure...