This report discusses the status and success of neutron therapy and some of the problems in clinical neutron dosimetry. Existing neutron interaction data, in particular results of kerma factor measurements and data evaluations, are reviewed. Nuclear data relevant for neutron source reactions, collimation, and shielding are also discussed. Finally, physical aspects of the variation of biological effectiveness of neutrons with neutron energy (radiation quality) are set out. Exchange of information between neutron therapy centers is essential, since only clinical experience can determine the optimal absorbed dose, fractionation, target volume, and clinical indications/contra-indications for neutron therapy. Refs, 44 figs, 19 tabs.