Cells are the basic organizing units of living material. Microorganisms can consist of only one cell but all larger organisms are multicellular. However, all complex organisms start as one cell: the fertilized egg. This divides and its progeny divide and ultimately an organism can have some number like 10e13 cells. These cells are organized into organs and tissues; as they divide they develop differential properties that give the tissues their identity. Also, the tissues are constantly renewed in adult life, requiring more cells. Thus, there needs to be a plan of development of the various organs and the plan that has evolved is for the early embryo to start with cells that can develop into all the cells of the body and then to spin off cells that have a more limited potential. We call the cells at this stage, stem cells because they are branches (stems) of a program. For instance, there are stem cells that can only become blood cells or only nerve cells. Now the blood and the nervous system each consists of many cell types, so these stem cells have a potential to have many fates, but only those limited fates of a particular compartment of the body. Now comes the confusing part. The stem cells of the early embryo, able to develop into all cells, are called embryonic stem cells and the cells of particular lineages are called adult stem cells. The confusing part is that the adult stem cells, while present in adults to allow tissue renewal, are also present in embryos. Embryonic stem cells are found only in very early embryos. With that background clearly understood, we can talk about the politics, policy issues and publicity that have dominated the discussions of stem cells. We can also discuss the potential value of stem cells as therapies and as experimental tools.