The study of the behavior of radionuclides has been divided into four items. First, the fractionation of radionuclides in soils is considered, according to the soil fraction associated with the radionuclides. Second, radionuclide migration along the soil profile is studied. Third, the role of microorganisms is presented. Finally, radionuclide bioavailability and transfer into plants is considered. Knowledge of the behavior of radionuclides in soil can lead to countermeasures in case of soil contamination. The chapter explains some scientific and social applications of radionuclide concentration measurements in soils, such as dose assessment, earthquake prediction through radon measurements, and dating of soil cores and erosion. If the scientific literature is reviewed, environmental studies on the presence of radionuclide concentrations include in-depth discussions of the fractionation, vertical distribution, the influence of microorganisms, and the soil to plant transfer of radionuclides.