Thirteen years ago, cardiologists began testing a radical new treatment in people: infusing a mix of bone marrow cells, collected from a patient, into his or her heart. As the field has matured, the science is in flux: No one is sure how these cells might affect heart function, and there's much debate over which cell types are worthy of investment. In addition to a mix of cells drawn from marrow, researchers are considering mesenchymal stem cells, embryonic stem cells, and more. Even doubters hope that some type of cell therapy will eventually make a difference for many heart patients. But in the meantime, there are fights over why some trials haven't been replicated and whether the animal data supports work in people, as well as charges of sloppy science and worse.