Before considering the basis of embryo culture medium, it is worthwhile reflecting on its function. A detailed overview of embryo metabolism is given in Chapter 4; however, in brief, the embryo must satisfy changing demands for energy by consumption of nutrients from the external milieu (Lewis & Sturmey, 2015). In an in vivo setting, these needs are catered for in a dynamic manner by the secretions of the oviduct; in an in vitro situation, these requirements must be satisfied by the embryo culture medium. In addition to the provision of energy substrates, the medium must also satisfy basic physicochemical requirements. Primarily, the medium must facilitate buffering of pH in response both to changing environments to which the embryo is exposed and to excretion of metabolic waste products, notably lactic acid, which is released by cells with accompanying protons, causing pH to fall. Moreover, the culture medium must avoid inducing osmotic stress. One of the major consumers of cellular energy is the maintenance of intracellular ion composition, maintained through the action of ion pumps. Providing suitable osmolarity and pH are among the most basic requirements of any embryo culture medium.