Cryopreservation of human embryos is an important tool for in vitro fertilization (IVF) practice. Human embryos can be cryopreserved efficiently from the pronuclear to blastocyst stage. For day 2 or day 3 embryos, it is better practically to culture the thawed embryos further for a period of time before transfer in order to confirm that these embryos resume development. The selection method for post-thawed embryos for transfer directly affects the clinical outcome in terms of pregnancy and live birth rates. Two basic techniques have been employed for the cryopreservation of human embryos: slow freezing and vitrification. Slow freezing was used first and is still the most common method for cryopreservation of human embryos. Vitrification is a relatively new technique and has been introduced to cryopreserve human embryos in recent years. Many studies have reported improved success rates in terms of clinical pregnancy and live birth rates with the vitrification technique.

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.