There is growing interest in the (nano−) scale structural analysis of condensed matter to study synthetic and biological polymers, colloids, porous materials, etc. Over the past two decades, small–angle neutron scattering (SANS), based on the availability of high fluxes of cold neutrons (wavelengths 4−20 Å), has proven to be one of the most important tools for such investigations. This success is due to a fortuitous combination of several factors of cold neutrons: high bulk penetrating power, the ability to manipulate local scattering amplitudes via isotopic labeling or an appropriate choice of solvent (contrast variation), minimal radiation damage, and small absorption for most elements. SANS has complemented and extended the information available from familiar small–angle X–ray scattering (SAXS) technique, and in parallel, there have been significant developments in SANS instrumentation (e.g. area detectors, tunable monochromators) and also in theory (e.g. mathematical modeling).