Secondary recovery is a process in which reservoir fluid is mobilized and moved from an injection well toward a production well. The success of this process greatly depends on the knowledge of reservoir continuity and uniformity, in terms of fluid transmissibility, and how much of the reservoir fluid volume can be contacted by the injection fluid. In any water/gas flood injection project, fluid channeling through mini-fractures, faults, and high permeability streaks results in problems such as poor reservoir sweep efficiency and low hydrocarbon recovery. Therefore, knowledge of direct communication between the injection and production wells as well as an understanding of formation heterogeneity can be of great help to overcome these problems. While techniques such as seismic, mapping geological deposition and reservoir simulation provide valuable information about the feasibility of secondary recovery projects, tracer testing is the only available method that provides valuable information on direct communication, flow-path, and formation heterogeneity across the injection and production wells. This paper presents a detailed review of chemical tracer applications in IOR with a supportive case history from a water-flood field. The paper also presents interpretation and discussion of the results on direct communication identification, formation heterogeneity evaluation, and swept pore volume calculation.