The X-ray diffraction technique has greatly improved our understanding of the function and properties of filamentous systems of biological and organic polymers by providing structural information at a molecular level. Although X-ray diffraction remains the primary method for determining fiber structures, neutron diffraction is an important supplementary technique. Neutron diffraction has some distinct advantages over the X-ray technique for providing certain types of structural information. This review describes the results from neutron diffraction studies of fibers and discusses the future prospects for this emerging technology.