This chapter discusses the radiotracer methods in in-situ techniques that use a radioactive label to monitor molecules of interest adsorbed at the electrode-solution interface. By pressing the electrode down against the scintillator, the detector responds to radioactive species adsorbed onto the electrode surface. Two main measuring principles are currently used for the radiotracer study of adsorption events at electrodes. Correspondingly, two different types of radiochemical-electrochemical cells are in operation. Both methods have been used for study of reversibly adsorbed and strongly chemisorbed systems. There are two major limitations in using thin-foil electrodes, namely, the restriction of the radiotracer work to polycrystalline electrodes and the need for electrode roughening, especially for weakly adsorbing species at low coverages. The smaller values of this ratio for higher concentrations show explicitly the advantage of the thin-gap method over the thin-film technique, making the latter one the most appropriate for studies of electrode adsorption on smooth surfaces.