The process of freezing sperm, eggs or embryos in extremely low temperatures (-196°C). This technique has been used for decades to freeze sperm. It allows us to quarantine the sperm while the donor is tested for transmittable infectious diseases. The sperm can then be thawed when necessary and used in artificial insemination. More recently, we have been able to freeze and preserve human embryos. This has enabled us to freeze and save embryos resulting from in vitro fertilization. These embryos can also be later thawed and transferred back to the uterus. This allows us to transfer fewer embryos at any one transfer, reducing the risks of high-order multiple births.