The infarcted heart heals by scar formation, and large myocardial infarctions typically result in heart failure. Although adult stem cells with the capacity to transform into various cardiac cell types and to secrete cardioprotective cytokines have been identified, endogenous repair mechanisms in the adult heart are not sufficient for meaningful tissue regeneration. These observations, however, suggest that it may be feasible to develop interventions aimed at enhancing these processes, and to promote functional and, eventually, structural recovery of the infarcted heart. Recent randomized clinical trials indicate that intracoronary delivery of bone marrow (stem) cells leads to an improvement in systolic function after myocardial infarction, thereby providing the first evidence that cell therapy may work in patients...