AbstractThis review is concerned primarily with the Lagrangian approach to mixing. First, the concepts of residence time and residence time distribution are introduced and defined. After a general treatment of this topic in the second chapter, the concept of micromixing is discussed in the third chapter with due emphasis on the application to chemical reactors. The final chapters extend the theory and applications to unsteady stale systems and to systems which are non-isothermal or non-homogeneous.