Many classical biochemical methods are available to study the primary metabolic pathway such as krebs cycle, glycolysis and HMP shunt pathway in living organisms. Number of advanced techniques has been developed to perform the bio synthetic study. The present review deals with the tracer technique and its applications in synthesis of secondary metabolites in pharmacognosy. This technique involves the stable (1H2,6C13,7N15,8O18) and unstable (1H1,6C14) radioactive isotopes. These isotopes are in corporate into presumed plant precursors with which they form a chemical bond and can be easily detected by various methods like liquid scintillation, giger muller and auto radiography techniques. These isotopes are regarded as radioactive markers in biosynthetic studies.