This chapter summarizes the technical challenges that had to be overcome for the freezing/thawing of intact ovaries with animal experiments first and then with humans. Two main problems with whole organ cryopreservation and re-transplantation have caused technical difficulties: the first related to the feasibility of executing a perfect vascular re-anastomosis of the whole organ with re-establishment of a prompt vascular flow; the second related to the development of a successful cryopreservation protocol. To improve the technique of freeze-thaw and transplantation of whole ovaries, adult female sheep have become the preferred animal model to study both slow cooling and vitrification methods. In the future, patients with malignancies at high risk of ovarian metastasis could have a whole ovary removed and perfused in vitro, to stimulate folliculogenesis in vitro. If successful, oocytes could be harvested for cryopreservation or for fertilization and subsequent embryo cryopreservation.