The spatial and temporal ranges accessible using the technique of quasi-elastic neutron scattering (QENS) are ideally matched to the atomic and molecular vibrational displacements, correlation lengths and diffusive motions encountered in highly complex biological systems. The QENS method has been successfully applied to a diverse range of bio-molecular problems which encompass, for example, proteins, membranes, lipids, nucleic acids and saccharides. In this section, the basic principles of quasi-elastic neutron scattering pertinent to the study of dynamic processes in biological molecules are presented. An overview of the neutron instrumentation required for such studies is given as are experimental results which highlight the ideas outlined.