The availability of cryopreservation technology has extended the scope of human-assisted conception treatment and made it more convenient for patients. Freezing of oocytes and ovarian tissue is now taking place in research institutions and should hopefully become widely available for clinical use in the future if the women have to be away at a critical time during the treatment; the frozen oocyte was thawed and used when required. Good quality embryos frequently remain after the transfer of the required number into the woman; these can be frozen and used at a later date, if required, instead of the couple going through another cycle of in vitro fertilization (IVF) treatment. The conduct of cryopreservation in humans is based on information originally derived from animal work, where sperm cryopreservation, for example, has been carried out for several decades for animal breeding. This chapter will consider the principles of cryopreservation followed by a general description of the practical steps. The clinical applications of cryopreservation will also be described. A final section will deal with frozen embryo replacement.