Stem cells possess an unlimited potential to regenerate and can acquire the identity of various differentiated cells. In the case of embryonic stem cells all the different cells of an organism can be generated. Recently the hitherto underestimated differentiation potential of adult stem cells has also taken center stage. These findings have opened up new perspectives in transplantation medicine and cell therapy for degenerative diseases. Before these possibilities can be realized, however, a number of ethical and scientific-technical obstacles have to be overcome. The latter are caused for example by the low abundance of the cell material as well as by possible immunological rejection reactions.