This publication provides estimates of the global amounts of radioactive materials generated in the first half-century of nuclear activities by both civilian applications of nuclear technologies and defence programmes. Despite the great progress achieved in many areas, some uncertainties remain, arising from the differences among classification systems used by various countries. Official information about radioactivity in contaminated sites as a result of accidents or weapon testing is even more incomplete. The resulting estimates, which are based on broad simplifications, are therefore characterized by unavoidable uncertainty. However, the estimates may be used for comparison with environmental burdens created by other means of energy production and other human activities.