The development of atomic industry and nuclear power engineering was accompanied by radiation incidents and catastrophes, some of these being rather large and resulting in radioactive contamination of the environment. Among the most important radiation accidents which have affected expanded areas are: the accident in the USSR in the South Urals with the formation of the East Urals Radioactive Trace in 1957, the accident in Sellafield, the United Kingdom in 1957, and the catastrophe at the Chernobyl NPP in 1986. The resulting radioactive traces – Emergency Radioactive Zones (ERZ) – have become the test sites for large-scale long-term radioecological investigations. The unique natural peculiarities of ERZs (wide variations in the environmental conditions – soil, plant-animal life, climatic factors, etc.) considering a wide range of man-made radionuclides (primarily biologically important 90Sr, 131I and 137Cs) released into the environment have made it possible to quantify the parameters of radionuclide migration via a large number of trophic chains in different natural environments (terrestrial, aquatic and other ecosystems). Information has been collected on radiation and post-radiation effects in various biota representatives in a wide range of doses and dose rates (especially at the population and ecosystem levels). Different countermeasures have been developed and applied on large territories for reducing exposure of the ERZ population and for remediation of the affected areas.