In this report the more general considerations of environmental behaviour of several radionuclides are discussed, including sources, transport to man and dosimetry. The radionuclides discussed are those most frequently released from natural and man-made sources and the greatest contributors to population radiation exposure under normal circumstances. Radionuclides are a special class of environmental substances. They are the unstable configurations of chemical elements which undergo radioactive decay, emitting radiation in the form of alpha or beta particles and x or gamma rays.