[en] Monoenergetic neutrons below energies of 20 MeV are most conveniently produced by reactions between the hydrogen isotopes or between protons and ⁷Li. By proper choice of reaction type monoenergetic neutrons up to 20 MeV can be produced with negligible secondary background radiation. The bombarding energies needed to initiate the reactions are modest and can be provided by electrostatic generators. The most common reactions are ⁷Li(p,n), T(p,n), D(d,n) and T(d,n). Care has to be taken in experiments to keep the neutron background as low as possible. This background can originate from secondary reactions, from breakup neutrons, or from the excitation of higher levels in the case of ⁷Li(p,n). These reactions cannot provide monoenergetic beams between about 8 and 13 MeV and in this 'gap' region inverse reactions are most favourable. The most practical way of producing quasi-monoenergetic neutron beams in the energy range from 20 to 100 MeV is by the bombardment of light elements with protons. However, it is not possible to produce strictly monoenergetic neutrons because the proton energy always exceeds the thresholds for particle breakup and the formation of excited states in the product nucleus. The most favourable targets for neutron production are generally considered to be ²H, ⁶Li, ⁷Li and ⁹Be. Because of the relative simplicity of manufacturing suitable isotopically-pure targets and the large 0 deg cross section, the ⁷Li(p,n) ⁷Be reaction (Q = -1.644 MeV) is probably the most practical source of quasi-monoenergetic sources over a wide range of energies. The main disadvantage of this reaction is that the main peak contains two unresolved neutron groups leading to the ground and 0.429 MeV first excited state in 7 Be, which limits the energy resolution to about 0.5 MeV. (author)