Stem cells are the closest thing science has to a fountain of youth, which helps explain the hope—and hype—surrounding them. Technically, a stem cell is one that, when it divides, can make not only a copy of its immature self but also more specialized daughter cells. The body has hundreds of types of stem cells. Some give rise only to a few types of cells—neural stem cells, for example, develop into different cells of the nervous system. More powerful are so-called pluripotent stem cells, which are present in early embryos and can give rise to all the cell types in the body. Stem cells allow researchers from many disciplines to explore how cells grow and divide, how they move, how they decide what type of cell to be, and how they work together to develop from a fertilized egg into a whole organism. The research also holds the promise of understanding and ultimately treating diseases as diverse as diabetes, heart disease, and cancer.