Stem cells are special kind of cell from an embryo, foetus or adult, capable of renewing itself under certain conditions and of becoming specialised cells that make up the different tissues and organs of the body. They may be isolated from embryos, umbilical cords and adult tissues. Stem cells from embryos can treat many diseases but they are extremely dangerous and unpredictable. However, this power and versatility of these cells to produce different cell types has medical applications in cell therapies. Embryonic stem cell are harvested but this requires the destruction of human embryos, which is considered immoral and unethical, thus becoming contentious and giving rise to ethical controversies. However, stem cells isolated from adult tissues and created from skin cells addresses these issues. Most recently, they have been produced by reprogramming skin cells, an advance revolutionizing this field. Stem cell therapies have great potential. Stem cells provide an ideal model for studying the development of an organism and have the ability to replace damaged cells in the body that would otherwise not be replenished.