For more than a half a century, biologists have upheld the theory that in most mammalian species, oocytes are formed before or shortly after birth, but never in adulthood. This foundation of reproductive science has survived the rapid growth of new technology and knowledge and has remained virtually unchallenged until two recent papers were published by the group headed by Jonathan Tilly. The first paper claims that mouse germline stem cells (GSCs) replace ovarian follicles that have been rapidly lost through follicle death (Johnson et al., 2004). The second paper, recently published in Cell, proposes continuous immigration into mouse ovaries of GSCs derived from bone marrow (Johnson et al., 2005). How could so many investigators have overlooked these basic mechanisms for so long, or have they?