The use of stem cells for cardiac repair is based on their potential to become cardiomyocytes and thereby to restore the functional capacity of the failing heart. The concept of transdifferentiation, by which adult stem cells adopt the fate of the cells they are intended to replace, generated enormous enthusiasm, but it is increasingly recognized that the plasticity of these cells is rather limited. This is particularly the case of skeletal myoblasts, which remain committed to their myogenic lineage. Likewise, conversion of bone marrow-derived cells into cardiomyocytes is, at most, an exceptional and quantitatively limited event. These limitations do not preclude the possibility that adult stem cells could have protective effects on left ventricular function, possibly through a paracrine action. Tissue-resident cardiac stem cells were recently identified but significant hurdles will have to be overcome if they are to be used for therapeutic purposes. Consequently, much hope is currently being placed in embryonic stem cells which, provided they are appropriately precommitted during culture, can differentiate into cardiomyocytes. Their subsequent electromechanical integration into the recipient myocardium can contribute to the repair of the damaged heart.