Stem cells are capable of regenerating tissue cells. They have an important potential use in a wide range of therapies, especially as an alternative to organ transplantation, with the advantage that they can be derived from the patient and thus avoid rejection. Embryonic stem cells are potentially capable of forming all kinds of cells. Their use is controversial however, because they are derived from early embryos and because, if they were to match the patient, they would have to be obtained using the same techniques that could, in theory, be used to produce cloned individuals. This article discusses the uses and problems of stem cell research and therapy.