“Stem-cell transplants are already performed every day in Harvard-affiliated hospitals — and around the world,” says Harvard Stem Cell Initiative codirector David Scadden, professor of medicine at Harvard Medical School and director of the Center for Regenerative Medicine at Massachusetts General Hospital. So-called bone-marrow transplants, which transfer tens of thousands of cells of many different kinds to a patient, most critically transfer hematopoietic (adult blood) stem cells. The proliferative capacity of these cells is so great, says Scadden, that researchers have demonstrated in mice the regeneration of the entire blood and immune system from a single cell. Hematopoietic stem cells routinely save the lives of people with diseases such as leukemia, lymphoma, and immune deficiencies. Why is this promising area of research, with the potential to do so much good, so controversial? The seeming simplicity of the idea — that there are stem cells that can generate new cells — belies the complexity of the science and the ethical ramifications of its application. A special report from Harvard Magazine investigates the work that Harvard researchers are doing to understand all the ramifications of the stem-cell revolution.