[en] The first nuclear reactors at Sellafield went critical in 1951 and fuel reprocessing commenced shortly afterwards. As the nuclear programme expanded, reprocessing increased and there was an associated increase in discharges to the environment. An initial environmental monitoring programme was formulated on the basis of research and assessment of the likely behaviour of radionuclides. In addition to the routine process sources there were also incidents that gave rise to acute releases of radioactivity to the environment. Of key significance were: the Windscale fire, 1957; short-cooled fuel reprocessing, 1981; and discharge of contaminated solvent, 1983. All of these incidents added to the requirements for environments for environmental monitoring. The monitoring programme has evolved over a period of more than 50 years. (N.C.)