The discovery of the helical structure of DNA, the determination of the three-dimensional structure of proteins, and the unraveling of the structure of viruses are among the outstang achievements of structural biology that have contributed much to our understanding of the molecular basis of life. These elucidations of the architectural details of biological structures provide insight into such diverse biological processes as the biosynthesis of macromolecules, the generation and storage of metabolic and photosynthetic energy, the expression and transmission of genetic information, and the control and recognition of physiological processes. The scattering of slow neutrons from condensed matter provides information about the structure and dynamical properties of such matter on a submicroscopic level. The type of information provided is related to, but qualitatively different from, that obtained by the scattering of x-rays. The neutron is an uncharged probe that interacts with the atoms of matter through two forces: the nuclear force, and a weaker, but longer-range, magnetic force between the neutron's magnetic moment and the magnetic moments of the electrons in the atoms. Although most present neutron studies in physics deal with magnetic and dynamical problems, in studies of structural chemistry neutrons are used to locate hydrogen atoms and tomore » analyze charge distribution in chemical bonds. Most biological neutron studies deal with structural problems, such as the position of atoms in a moleculelike protein or the relative position of proteins, lipids, drugs, and other entities in larger biological assemblies such as membranes.« less