Cryopreservation protocols for isolated oocytes and complex ovarian tissues can be broadly classified as equilibrium freezing (slow-freezing protocols) or rapid freezing (vitrification protocols). Oocyte cryopreservation requires that the gametes tolerate three non-physiological conditions: exposure to molar concentrations of cryoprotective agents (CPAs); cooling to subzero temperatures and removal of or conversion of almost all of the liquid cell water into the solid state. Penetrating CPAs, such as glycerol, dimethyl sulphoxide (DMSO), ethylene glycol (EG) and 1,2-propanediol (PrOH), are all membrane-soluble and can pass into cells. Non-penetrating CPAs include sucrose, glucose, trehalose and polymers such as hydroxyethyl starch and polyvinyl pyrrolidone. The rate at which cells are cooled is fundamental to the success of cryopreservation. Consequently, at the time of writing, mature metaphase II (MII) oocyte cryopreservation remains a potential solution rather than a practical remedy for infertile women.