1 Neutron Scattering for Biology T.A. Harroun, G.D. Wignall, J. Katsaras 1.1 Introduction The structure and dynamics of a specimen can be determined by measuring the changes in energy and momentum of neutrons scattered by the sample. For biological materials, the structures of interest may be complex molecu- lar structures, membranes, crystal lattices of macromolecules (e.g., proteins), micellar dispersions, or various kinds of aggregates. These soft materials may exhibit various modes of motion, such as low-energy vibrations, undulations or diffusion. Neutrons are non-charged particles that penetrate deeply into matter. Neu- trons are isotope-sensitive, and as they possess a magnetic moment, scatter from magnetic structures. Neutron scattering can often reveal aspects of struc- ture and dynamics that are difficult to observe by other probes, including X-ray diffraction, nuclear magnetic resonance, optical microscopy, and var- ious spectroscopies. It is particularly powerful for the study of biologically relevant materials which often contain hydrogen atoms and must be held in precise conditions of pH, temperature, pressure, and/or hydration in order to reveal the behaviors of interest. Neutron scattering is practiced at facilities possessing reactor-based and accelerator-based neutron sources, and to which researchers travel to under- take their scattering experiments with the help of local scientific and technical expertise. Compared to traditional “hard” materials, in biologically relevant materials the characteristic length-scales are larger and the energy levels are lower. As such, additional neutron scattering measurements are possible if the reactor or accelerator-based source includes a cold moderator that emits a large proportion of long wavelength, lower velocity neutrons, which are better suited to the typical structures and dynamics found in bio-materials. This chapter will follow neutrons from their production in a fission or spallation event, into the specimen where they scatter and are subsequently detected in a way that discriminates changes in momentum and energy. The advantages of using neutron scattering for problems in biology will be outlined.