In recent studies, the formation of germ cells from ES cells in a cell-autonomous manner when LIF is depleted from the medium, and the generation of post-meiotic cells from ES-derived germ cells in long-term culture have been elucidated. These remarkable results indicate the possibility of producing gametes from ES cells in culture. Together with in vitro maturation of gametes (eggs and sperm), ES (or iPS)-derived germ cell research has the potential both to reveal the basic mechanisms of gamete biology and to overcome clinical problems. For example, ES-derived germ cells purified from various culture stages enable us to explore molecular mechanisms such as gene functions and epigenetic modifications, which are essential for gametogenesis and fertility in humans as well as in experimental animals. Also, in vitro gametes from pluripotent stem cells will allow us to develop new ways to treat infertility, and advances in this field may realize a novel system of the alternation of generations in vitro without formation of individual bodies. Thus, it seems that ES cell- or iPS cellderived gamete research is of incalculable scientific value.