It is a challenge to induce spermatogenesis in vitro, because it involves a complicated process of sequential cell proliferation and differentiation, from spermatogonial stem cells to sperm formation. Recently, we have succeeded, using a classical organ culture method, to produce sperm from spermatogonial stem cells, using several modifications. The produced spermatids and sperm were fertile, giving rise to healthy progeny. This new in vitro system for spermatogenesis could be useful for addressing the issue of sperm quality that is becoming more important in this ICSI era. We also hope that an in vitro system of human spermatogenesis will be developed in the near future.