Oocyte cryopreservation has long been envisaged as the ideal solution to the need to capitalize the reproductive potential derived from a cycle of ovarian stimulation, circumventing the ethical and legal problems posed by embryo cryopreservation. This chapter presents an overview of the current status of oocyte cryopreservation by slow-cooling methodology. It discusses recent advances, unresolved matters, and possible future developments. Cryoprotection may be achieved through the use of cryoprotective agents (CPAs), chemicals that interfere with the water-ice transition and interact with biomolecules, acting as water replacement. The use of alternative and less toxic CPAs is a potential approach in the development of more efficient slow-cooling protocols. Assessment of oocyte quality is one of the most crucial, intriguing and, yet, unresolved questions in vitro fertilization (IVF) and gamete biology. Ooplasmic vacuolization is frequently detected in mammalian oocytes treated with different cryopreservation protocols or exposed to CPA alone.