Historically each organ and tissue was thought to possess cells capable of self-renewal and of giving rise to a large number of differentiated descendents. These cells, termed adult stem cells, were believed to be restricted in their potential. This tissue specific restriction was accompanied by the belief that as cells differentiate, they lose their ability to make fate choices. This is exemplified by many well-characterized cell types, such as hematopoietic stem cells and neural stem cells. In the past few years several reports have suggested that such stem cells may possess developmental capabilities resembling those of more immature and potent cells such as embryonic stem cells. These findings are raising fundamental questions about the traditional hierarchical view of developmental biology and challenging the established beliefs and dogmas developed in biology over the past century.