This paper presents a review on residence time distribution (RTD) studies in continuous thermal processing of liquid foods. The theoretical basis of the Danckwerts analysis is summarized, as well as the most important flow models, with special emphasis on tubular systems. Methods for experimental determination, modelling and estimation of RTD are critically described. While main design objectives in continuous thermal processes may be guaranteed by a proper minimum residence or holding time, process optimization requires the knowledge of the residence time distribution. Both concepts are reviewed and discussed. A significant scatter was noticed among published results and the need for a systematic work is clear It was concluded that future research should focus on studies at pasteurization/sterilization temperatures, as well as ori studies conducted with real food products or model food systems with non-Newtonian flow behaviour. Furthermore, information relating RTD to processing conditions would be a useful tool for process optimization.