This review is limited to the principles and technical aspects of the ultra-high-temperature (UHT) sterilization process only. Theoretical basis of the UHT processes is illustrated using the first order kinetics approach with Dt, z, and Ft values. Recent information in the areas of process engineering, microbiology, enzymatic changes, quality, functionality and nutritional value of UHT products is presented. Among the current problems discussed are the burn-on and fouling of heat transfer surfaces, processing of heterogeneous mixtures, unusual strains of microorganism with very high heat resistance, heat-resistant enzymes of microbial origin, enzyme reactivation, and the effects of UHT processing on impaired functionality, final product quality, and chemical changes in individual food components.