The concept of residence time distribution (RTD) is an important tool for performance assessment of industrial units and reactors. This paper discusses the residence time distribution concept, how to obtain the residence time distribution curve of a process reactor using radioactive tracers (radiotracers) and its importance in process performance assessment. Focussing on the fluid catalytic cracking unit (FCCU) of a petroleum refinery, radiotracer tests to determine catalyst and vapour traffic velocities and slip through the riser and its implications for efficient cracking, and the flow distribution through the riser and regenerator are discussed.