Radioisotopes of both natural and artificial origin are widely used in environmental research and protection to tackle problems in which high specificity and sensitivity are required. The paper exemplifies the capabilities guided by some recent investigations in various fields of application: dating of river and marine sediments, determination of dry deposition rates for airborne particulates, studies of the uptake of organic compounds by plants and of the internal distribution processes in plants, investigation of transport phenomena in soil, groundwater or surface waters, development of trace analytical procedures, application of neutron activation analysis, detection of trace gases by means of ionization detectors, and the exploration of mud fields in tidal rivers using a radiometric density gauge.