Cryobiology is the core of fertility cryopreservation. The principal application for human fertility cryopreservation began with sperm freezing, and then developed to include embryo and oocyte as well as gonadal cryopreservation. This chapter briefly discusses the scientific background and the current basic knowledge of cryobiology. Aqueous solutions are important for cryobiology since the freezing of biological systems always involves solutions containing substances such as electrolytes, non-electrolytes, polymers, and so on. Some of the classic papers in the field of cryobiology describe the theories and the mechanisms of cryoinjury during cell freezing and thawing. Some cryoprotectants reduce the injury of cells during freezing and thawing. Today, the most commonly used cryoprotectants in the field are glycerol, dimethyl sulfoxide, ethylene glycol, and propylene glycol. Cryoprotectants can interact with each other in a mixture, or with crucial cell molecules, thereby producing effects other than those that would occur with an individual cryoprotectant.