In application to structure determinations, modern physical techniques can be subdivided into three groups of resolution, namely those at high, sub-atomic resolution, those at atomic resolution, and those at low, overall resolution. At the sub-atomic level, techniques such as nuclear magnetic resonance permit the microenvironment of single atoms within individual residues to be studied. At the atomic level, single crystal diffraction studies gives details of the molecular arrangement of the residues within the macromolecule, and for this reason, this is the most powerful structural technique available. Low resolution methods include not only X-ray and neutron solution scattering but also other related techniques such as total sequence determinations, electron microscopy, light scattering and hydrodynamic studies, all of which report on various aspects of the overall macromolecular shape. These complement powerfully the crystallographic method for a number of reasons, including those cases where the macromolecule cannot be crystallized or where it is required to work in a more physiological environment in solution. This chapter presents a survey of the use of both X-ray and neutron scattering methods in biological applications. © 1985, Elsevier B.V. All rights reserved.