Use of radiotracer in residence time distribution (RTD) studies in industrial as well as lab scale systems has been emphasized in this review. The advantages of radiotracer over conventional tracers are discussed. The injection and detection protocols of the radiotracers are explained. The pretreatment of RTD data obtained and RTD modeling are explained using industrial- and laboratory-scale studies. The detailed RTD studies in (i) oil and gas, (ii) pharmaceuticals and bioprocesses, (iii) fertilizers and pesticides, (iv) polymer, plastic and fiber, (v) mineral processing, organic and inorganic chemicals, (vi) laboratory scale, (vii) pilot plant scale, and (viii) wastewater treatment processes are considered for this review. The case studies are also explained with RTD experiments. The current trend on the application of radiotracer technology in industry is also explained.