I would like to discuss this evening some aspects of neutron physics in which the wave-like character of these particles is highlighted. According to the results of quantum mechanics or wave mechanics, all particles give rise to interference phenomena, similar to those of light radiation, in which the wavelength is given by de Broglie’s relation ( \(\uplambda = h\) /mv), that is to say the ratio of Planck’s constant to the momentum mv of the particle. For such phenomena to be observed, it is necessary that the wavelength be of the order of magnitude of other geometric dimensions contained in the problem; it is therefore convenient to indicate without fail, its value. For so-called thermal neutrons, that is to say neutrons that are slowed down to thermal motion velocities corresponding, at room temperature, to the energy of about 1/40 eV, it is found from de Broglie’s relation that the wavelength is about 1.8 \(\cdot 10^{-8}\) cm. Since this length is very close to interatomic distances, one expects to observe for neutrons diffraction phenomena in crystal lattices similar to the phenomena observed with X-rays, which also have wavelengths of this order of magnitude.