Heart regeneration via stem cell therapy could improve the functional outcome for millions of patients. A goal of cardiac stem cell research is to foster the engraftment of new, beating cardiac cells into the ischemic region of the heart after a myocardial infarction. The key elements of cell therapy for myocardial repair reviewed here are the source of cells and the mechanisms by which these cells improve cardiac function. Injection of stem cells into the heart of animals ignited the field by showing some functional cardiac improvement. Unfortunately, few injected cells are retained in the heart or become a new, beating myocardium, and clinical trials have shown moderate improvement of human heart function. The causes of the minimal functional improvement are still unknown, but blood vessel formation (angiogenesis) or secretion of growth factors or cytokines are likely candidates. Cells appropriate for human therapy might be mesenchymal stem and progenitor cells from bone marrow or the heart itself. A more controversial cell source, embryonic stem cells, have a nearly unlimited self-renewal potential and can differentiate into beating cardiac myocytes. However, all of these cell sources and the mechanisms of improvement need further research, with the differentiation of stem cells into functional cardiac cells a difficult but most beneficial hurdle to leap.