The potential usefulness of human embryonic stem cells for therapy derives from their ability to form any cell in the body. This potential has been used to justify intensive research despite some ethical concerns. In parallel, scientists have searched for adult stem cells that can be used as an alternative to embryonic cells, and, for the heart at least, these efforts have led to promising results. However, most adult cardiomyocytes are unable to divide and form new cardiomyocytes and would therefore be unable to replace those lost as a result of disease. Basic questions — for example, whether cardiomyocyte replacement or alternatives, such as providing the damaged heart with new blood vessels or growth factors to activate resident stem cells, are the best approach — remain to be fully addressed. Despite this, preclinical studies on cardiomyocyte transplantation in animals and the first clinical trials with adult stem cells have recently been published with mixed results.