In vitro/ex vivo egg production has been widely studied in various mammalian species over half of the century to utilize the majority of the immature oocytes stocked in the female ovaries. Recently, the first successful protocol of in vitro oogenesis from primordial germ cells (PGC) has been established by Morohaku K, et al., resulting in the live birth of offspring in mice. The protocol consists of two vital steps; 1) ex vivo organ culture of mouse PGC ovaries to complete the process of follicle formation, with successful incorporation of antagonists for the existing estrogen receptors, and 2) in vitro follicle culture of the growing follicles isolated from the cultured ovaries. The review in this issue introduces the current findings and aspects governing in vitro oogenesis, with a brief history (Morohaku K. A way for in vitro/ex vivo egg production in mammals. pp. 281–287).