A survey is given of some current applications of neutron diffraction to the elucidation of a wide range of the properties of both magnetic and non-magnetic materials. Naturally, one must use quantum mechanics in obtaining the details necessary for the interpretation of experimental results, though some useful semiclassical visualization is possible. The analysis of the dynamics of atomic systems into elementary, nearly independent excitations started with Debye's analysis of vibrational modes in 1912, and this gave rise to the quantized unit of vibrational energy, the phonon. Likewise, magnetic spin-wave theory started with Bloch in 1932 and in its quantized form gives us the ‘magnon’ concept. In complex structures, in liquids, and near the critical point theoretical concepts are less clear and we may use the neutron experiments to help us decide on the most appropriate approximation to use in describing them. We now seem to have a tool of remarkable power to help in solving the complex problems of energy levels and the dynamics of crystals.