Stem Cells are characterised by their two properties of capability of renewing themselves and differentiating into a diverse range of cell types. The two broad types of mammalian stem cells are: embryonic stem cells and somatic or adult stem cells. Embryonic stem cells are isolated from the inner cell mass of blastocysts, while adult stem cells are found in adult tissues. In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, dividing essentially without limit to replenish specialized cells. Because of their pluripotency and unlimited capacity for self-renewal, embryonic stem cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease. Reprogramming of somatic cells with defined factors can be a resolution to the problem of allogeneic stem cell transplantation in embryonic stem cell therapies. Adult cells, which have been genetically reprogrammed to an embryonic stem cell–like state by being forced to express genes and factors important for maintaining the defining properties of embryonic stem cells, are referred to as induced pluripotent stem cells (iPSCs).