Stem cells are defined as cells with the ability for self-renewal and differentiation. Hematopoietic stem cells are well known, and their application is useful for the treatment of various kinds of diseases. Recently, neural stem cells have been identified even in the adult brain, which has up to now been considered to be a tissue with no regenerative capacity. In addition, it has emerged that tissue stem cells can differentiate into various kinds of cells beyond their original characteristics. Here, we discuss the self-renewal mechanisms of embryonic stem (ES) cells, hematopoietic stem cells and neural stem cells.