Chemical speciation for a few elements can be facilitated to a great extent by incorporating a suitable radioisotope into the system and measuring the radiation of the isolated species. This radiospiking can be applied to in vitro and in vivo labelled experiments. Radionuclides are, however, also present as an anthropogenic contaminant from various nuclear fission activities. The radiotracer should be added under such conditions that it behaves in exactly the same way as the isotopes it represents. It should possess an adequate radioactive half-life, and preferably be a gamma-emitter because of the ease of detection. Radiotracer labelling is now widely used to study speciation problems of many essential and toxic elements in body fluids and tissues. It can be used to trace the different locations where the element is metabolized and stored, and subsequently to detect the element in the isolated biocomponents. The determination of the location of a radiotracer in a cell by autoradiography proved to be impractical because of the lack of resolution. Radiochemistry is similarly very useful for investigating particular aspects of the speciation of heavy metals as they occur in the ecosystem, and to follow the fate and effects of fission nuclides in the environment as they are carried around by the water and air masses. However, in certain circumstances the behaviour of fission products appears to be different from that of their stable analogues. For the actinides they simply do not exist. Radiochemical methods are a major tool for identifying and quantifying the nuclides in the different species.