Radiolabeled test substances are used in studies designed to identify and quantify the breakdown products of agrochemicals in plants, livestock, soil, and water to understand their fate in the environment. This chapter introduces the term radioactivity and presents a brief discussion on the properties and rates of radioactive decay and explains the basic units used to quantify radioactivity in test samples. The difference between nuclides and radionuclides, the nature of radiation originating from disintegrating radionuclides and the characteristics of radiation produced by alpha and beta particles, and gamma rays are also presented. The safe handling of radioactivity and regulation of its use by the government agencies are also briefly discussed in this chapter. Special emphasis is given to radioactive 14C radionuclide, which is preferentially used in agrochemical tracer studies.