Specimens are frozen to provide a hardened matrix for sectioning and to preserve the morphological, biochemical, and immunological properties of cells and tissues. Freezing tissue specimens has the potential to eliminate many problems associated with standard practices of chemical fixation and paraffin or resin embedding. In practice however, freezing can dramatically alter the physical and chemical structure of cells and tissues and cause the formation of ice crystals. Cryoprotectant agents control the rate of cooling in order to limit the formation of ice crystals. Optimal Cutting Temperature (OCT) is a routinely used water-soluble glycol and resin cryoprotectant agent that provides an excellent specimen matrix for cryostat sectioning at temperatures of -10\u00b0C and below.