Regenerative medicine and artificial organs are fields of development in the 21st century. Various stem cells are found in many organs, and it is expected that medical treatment with the induction of regeneration in vivo and ex vivo using growth factors will be available. On the other hand, human embryonic stem cell lines (ES cells), which can transform themselves into any cell in the body, have been established. Since the development of cloning technology, the production of "individual ES cells" is also no longer only a dream. At the same time, the use of human cloned embryos entails important ethical and social considerations. As is well known, although artificial organs are not perfect, they can substitute for the functions of the kidney and heart. In the future, miniaturization and advanced features can be expected by application of microtechnologies. To execute various functions of synthesis and metabolic processes, for example, with an artificial liver, hybrid artificial organs incorporating cell functions are in development. We should not use animal cells because of unknown infectious diseases. It is therefore desirable to use human cells by the application of regenerative medicine or genetic engineering. Because both regenerative medicine and artificial organs can be expected to change medical treatment dramatically in the 21st century, steady efforts to clarify cell functions and molecular mechanisms will be required, as well as addressing ethical/social considerations.