[en] A general discussion is given of the use of neutron reaction data in the nuclear energy programme and of the importance of accurate fission cross-section data. A brief summary of the neutron physics of nuclear reactors is followed by a survey of the types of fuel cycle and the fission cross-sections which are of principal interest. The use of nuclear data in reactor calculations is examined and typical target accuracy requirements for the prediction of reactor properties are summarised. Simple arguments are given to show how the accuracy of prediction of some important reactor properties depends on the accuracy of the nuclear data. The use of integral measurements to supplement energy dependent cross-section data is reviewed and a procedure outlined which has been developed for defining a combined programme of integral data and differential cross-section measurements which could meet the chosen target accuracies for a set of reactor parameters. Finally typical fission cross-section accuracy requirements are presented. (U.K.)