Low-energy neutrons are particulary well suited to probe crystalline, glassy and biological materials. The de Broglie wavelength of a thermal neutron is of the same order as interatomic spacings, and its anomalously high cross section for scattering off hydrogen nuclei makes it an especially useful probe for hydrogenous substances. Furthermore, because its electromagnetic interactions are limited to its magnetic dipole moment, the slow neutron has a very large penetration depth with little disruption of the sample under study, and it serves as an excellent probe of complex magnetic structures.