Ultra-high-temperature (UHT) processing today may describe either a sterilization or a pasteurization process. This review primarily covers the sterilization process which involves heating milk at 130 to 150 C for 1 second or more and then packaging it aseptically. The main topics covered are: history, economics. and theoretical basis of UHT processing; common UHT processes and available aseptic packaging systems; microbial, nutritional, biochemical, and physical aspects of UHT milk; and the flavor of UHT milk.