The use of small-angle neutron scattering (SANS) for the study of biological macromolecules is described. Topics covered in this chapter include: the theoretical basis of SANS; contrast variation; and the measurement of distances between chemically specified points in a macromolecule or macromolecular complex. Practical considerations are addressed and a number of examples are provided. Keywords: contrast matching; contrast variation; distance measurements; internal contrast variation; isotopic substitution; molecular weight; small-angle neutron scattering; spin contrast variation; triple isotopic substitution