The regulation of mammalian oogenesis in vivo is complicated because of numerous constantly changing events caused by ovarian cells interacting with or influencing each other. One of the most intractable questions for nearly the last 80 years has been the mechanism controlling the maintenance of meiotic arrest and the resumption of oocyte meiosis in a pre-ovulatory follicle. The question is now mostly resolved, as the regulatory mechanisms of cGMP, cAMP, and the NPPC/NPR2 system in the follicle, have recently been uncovered. Oocyte growth in vitro has also been the subject of extensive research utilizing growing oocytes at various stages in several species, including mice, cattle, pig, sheep, goat, and horse. Remarkably, the first reconstitution of the entire process of mammalian oogenesis in vitro from primordial germ cells (PGCs) was recently achieved in mice. Furthermore, even PGC-like cells, originally produced from mouse embryonic stem cells and induced pluripotent stem cells, can develop into functional oocytes in vitro with the help of gonadal somatic cells of female mouse fetuses. These updated findings and newly developed culture systems will assist in gaining a better understanding of the mechanisms of oogenesis and will also lead to the creation of new gamete resources for mammals.