The use of stem cells for cardiac repair is a promising opportunity for developing new treatment strategies as the applications are theoretically unlimited and lead to actual cardiac tissue regeneration. Human embryonic stem cells were only recently cloned and their capacity to differentiate into true cardiomyocytes makes them in principle an unlimited source of transplantable cells for cardiac repair, although practical and ethical constraints exist. Also, the study of embryonic stem cells and their differentiation into cardiomyocytes will bring forth new insights into the molecular processes involved in cardiomyocyte-development and -proliferation, which could lead to the development of other strategies to augment in vivo cardiomyocyte numbers. On the other hand, somatic stem cells are alternative cell sources that can be used for cell transplantation purposes. They do not evoke ethical issues and bear less ethical constraints. However, they also appear to be much more restricted in their differentiation potential than the embryonic stem cells. Here we discuss the use of both cell types, embryonic and somatic stem cells, in relation with their importance for the clarification of cardiomyocyte-development and their possible usefulness for clinical therapy.