[en] Since the time when neutron radiation became available, its biological effects have been intensively studied for a number of reasons: For a long period neutrons alone constituted a densely ionizing radiation with high penetrating power in matter. Therefore, neutron radiation is a particularly important tool of fundamental radiobiological research. Neutrons are of great practical importance in radiation protection and nowadays also in radiation therapy. Owing to these facts, fast neutrons became a main subject of experimental and theoretical exploration of the problem of relative biological effectiveness (RBE). As to the latter question, it is shown that simplified physical concepts of radiation quality alone (LET, microdosimetry) are unable to provide a satisfactory understanding of RBE. Starting from biophysical considerations (DNA lesions, cell damage, refined stochastics of energy deposition), however, one can construct a framework yielding a more complex RBE theory which also takes account of radiobiological properties of the special object and its reaction under study. Applications of this theoretical formalism with respect to experimental results at the cellular level, with emphasis on fast neutron radiation, are discussed, and the relevance of the subject to radiation protection and, in particular, neutron therapy is pointed out. In the latter respect it is argued that taking advantage of the specificity of effectiveness of neutrons on different tissues requires theoretical understanding of RBE and its determinants, including the possibility of quantitative predictions. (author)