The basis for therapy with fast neutrons is discussed and three fields are considered: The physical field, the physico chemical field, and the biological field. In the physical field no advantages in the dose distribution, in the influence of scattering and in the consideration of inhomogeneities in the irradiated tissue are found. In the physico chemical field, the marked advantage of fast neutrons is an avoidance of the 'oxygen effect', which must be expected under sparsely ionizing radiation. In the biological field one can expect lower reparation under irradiation with fast neutrons than with sparsely ionizing particles. Therefore, the difference in action between tumour and healthy tissue diminishes under fractionated irradiation and calls this to one's attention in practical therapy.