Within recent years studies of the natural distribution pattern of the radioactive elements have been developed as a prerequisite to explorations for the raw materials of atomic energy. One important result of this activity is an improvement of our geochronological knowledge of the stratigraphical succession. Memorable bench-marks are Lower Cambrian at 600 million years, Upper Carboniferous at 300 m.y., Middle Cretaceous at 100 m.y. So-called Proterozoic rocks may locally be more than 2,000 m.y. old (Witwatersrand System) and may pre-date several orogenies in so-called Archaean rocks elsewhere. Some ore deposits provide factual evidence for theories of reworking: for example, ores apparently of late Mesozoic age can derive from Hercynian granites. Radiogeological studies now play an important part in prospecting for ore deposits and for coal and oil fields, and they can be of much significance in evaluating the health hazards arising from artificial radioactivity.