The ovaries are very sensitive to cytotoxic treatment, especially to alkylating agents. It is clear that high doses of alkylating agents, irradiation, and advancing age all increase the risk of gonadal damage. This chapter presents the oncological indications for ovarian tissue cryopreservation. Cryopreservation of oocytes can be performed in postpubertal patients who are able to undergo a stimulation cycle, but the effectiveness of this technique is still low, with delivery rates from 1 to 5% for frozen-thawed oocytes using the slow-cooling techniques. The main drawback of ovarian tissue cryopreservation followed by avascular transplantation is that the graft is completely dependent on neovascularization and, as a result, a large proportion of follicles are lost during the initial ischemia occurring after transplantation. Reducing the ischemic interval between transplantation and revascularization is, therefore, essential to maintaining the follicular reserve and extending the lifespan and function of the graft.