The concept of residence time distribution (RTD) is an important tool for the analysis of industrial units and reactors. The RTD of fluid flow in process equipment analyses their performance and checks the feasibility of a reactor. RTD method has been widely used in industry to optimize processes, solve problems, improve product quality, save energy, and reduce pollution. The technical, economic, and environmental benefits have been well understood and recognized by various sectors such as industrial and environmental sectors. The petrochemical industries, mineral processing, and wastewater treatment sectors are identified as the most appropriate target beneficiaries. This review traces current applications of the residence time theory in various chemical industries. Besides reviewing recent experimental studies in the literature, some common modeling, tracer injection and detection technique, and different parameters are studied to understand RTD.