[en] The problems associated with the nuclear reactions in a fusion reactor are discussed. The reactions considered are D-T, D-D, and D-3-He reactions. The most promising reactor is a D-T reactor. The data concerning neutron-induced reactions are required from the view points of tritium breeding, heat generation and irradiation effect. The Li-n reactions are important for tritium breeding. The accuracy of the cross-section measurement of the reactions has some effect on the breeding ratio, and the precise measurement is requested. The (n, 2n) reaction with the vacuum wall material and blanket of a fusion reactor contributes on the tritium breeding. The neutron gain with the (n, 2n) reaction in Steiner blanket is 0.075+-7%. The important causes of heat generation are nuclear reaction and gamma-ray. For the evaluation of these causes, the distribution of heat sources is necessary. The data concerning the distribution are scarce, and the measurement is required hereafter. The results of calculation by Steiner on the heat generating rate showed that the rate was large at the vacuum wall, and the difference was seen between the designs of reactors with Li coolant and with coolant of flibe. The radiation effects on materials by neutrons are due to atomic displacement, nuclear transformation, and hydrogen and helium production. It is necessary to measure the reaction cross-section concerned. However, the measurement of the neutron energy between a few MeV and 14 MeV is hard because of the lack of neutron source. The estimation by calculation is in progress. (Kato, T.)