SynopsisThe background to recent major advances in the construction and operation of fast reactors is outlined with particular reference to the Dounreay Prototype Fast Reactor. The advantages and disadvantages of sodium as a coolant of the high energy density assembly are discussed and an account given of the consequences of a leak and the precautions taken against this eventuality. Attention is drawn to the safety aspects of the system. The economics of the plans for fuel reprocessing are explained and an account given of the progress in the fabrication of fast reactor fuel pins. Finally the environmental impact of present and planned activities on the Dounreay site is presented in the context of participation in the European Collaboration on Fast Reactor Technology and attention drawn to the importance of the planning inquiry in progress at Dounreay.