Embryonic stem (ES) cells, derivatives of cells of early mammalian embryos, have turned out to be one of the most powerful tools in developmental and stem cell biology. When injected into embryos, ES cells can contribute to tissues derived from all three germ layers and to the germline. Amazingly, ES cells in culture are able to recapitulate features of embryonic development spontaneously. In addition to previous successes in deriving somatic cell types, recent studies have shown that both mouse and human ES cells can also give rise to primordial germ cells (PGCs) in culture. These mouse germ cells appear to be capable of undergoing meiosis and forming both male and female gametes. Although the full function of these ES-derived germ cells and gametes remains to be demonstrated, these findings open the door for undertaking new types of reproductive studies and novel approaches in regenerative medicine.