**Abstract**

The biochemical and metabolic activities of living cells are virtually stopped at ultralow temperature and they enter into a suspended state of animation. However, as such, exposure of living cells to ultralow temperature is associated with complex changes that reduce their survivability following freeze-thawing. Cryopreservation is the method for preserving living cells at ultralow temperature at genetically and physiologically stabilized state. Cryopreservation of oocytes and embryos is an integral part of the assisted reproductive technologies with many potential applications. Cryobanking of oocytes and embryos derived from genetically superior animals is promising for enhancing the outcome of planned breeding programs and conserving biodiversity of endangered animal species. Cryobanking can also ensure steady supply of oocytes and embryos for many downstream applications of assisted reproduction such as in vitro embryo production, embryo transfer, production of stem cells, and genetic engineering. Tremendous advancements have been made in this field over the past 5 decades and several methods have been demonstrated for cryopreserving oocytes and embryos in different species. This chapter focuses on the fundamental aspects of oocyte and embryo cryopreservation.

**Keywords:** cryopreservation, oocyte, embryo, methods, challenges
