The editors take pleasure in presenting a further vol ume in their annual review series. the present volume con tains six papers that may be said to span from the theory of design to the practice of operation of modern nuclear power stations, therefore concentrating on nuclear energy as a source of electrical power. starting with the most mathem atical, and proceeding in the direction of technology, we have the chudley and brough account of a new interpretation of (linear) boltzmann transport theory in terms of the characteristic or ray approach. this seems to be new in application here, but of course the method is the child of many classical studies in the solution of partial differen tial equations and proves to remarkably well-suited to modern computers and their numerical bases. we might put the article by dickson and doncals on the design of heterogeneous cores next, with its significance for fast reactors of the future. the various "central worth" discrepancies, with their implication for safety and relia bility founded on, inter alia, the doppler effect, have made this a major area for resolution: to see that we can develop design methods and codes that will reconcile theory and exper,. . . iment to the point at which theoretical designs could be accepted for building without the need for a full-scale mock up, as had to be done in the 1950's for the light water re actors.