For their unique properties stem cells promise to be of universal use in clinical medicine, especially in regeneration of many organs and tissues in the human body. This attractive subject receives an ever growing attention of specialists from different branches of science and, no doubt, will present one of the most studied trends in medicine in the new millennium. In this communication, the authors discuss two main sources of human stem cells potentially suitable for cell-based therapy. The first are the cells obtained from embryonic tissues--embryonic stem cells, the second are the cells derived from adult tissues--adult stem cells. Presently, harvesting and therapeutic use of embryonic stem cells are associated with many problems both methodical and ethical. Utilization of adult stem cells in cell-based therapy is a certain solution in the current state of replacement therapy. Still, we have to be aware that this is not a compromise but one of the most prospective ways to treat a variety of serious diseases. To date, it is not yet clear which way would be more suitable and it is up to us which way we choose for the benefit of millions of patients. Considering the current state of knowledge, it is impossible yet to predict which stem cells--embryonic or adult--or therapeutic approaches would yield the best results. Much research is to be done and verified in practice and, at the same time, ethical problems must be resolved.