Since their first discovery, embryonic stem (ES) cells have been the focus of intense research and public scrutiny. Derived from discarded 5-day old embryos (typically obtained from patients undergoing in vitro fertilization), this type of stem cell possesses the ability to create any tissue in the human body and replicate indefinitely in culture conditions. Because of these properties, ES cells represent a unique research opportunity to understand human development and create new therapies for disease.