Small-angle neutron scattering (SANS) is a technique that literally deals with “scattering at small angles” and has been developed as a method for investigating nanoscale structures. While its theoretical background is largely the same as small-angle X-ray scattering (SAXS), SANS is highly valued for its excellent material penetration and the ability to selectively visualize structures using techniques like deuterium labeling. In this chapter, we’ve strived to express the scattering intensity distribution in a unified and consistent manner as the Fourier transform of the scattering length density distribution. The subjects cover range from dilute particle systems like colloidal dispersions to semi-concentrated and bulk systems, where evaluating interactions is crucial. Furthermore, the chapter extends beyond particle systems to include non-particulate systems such as single-phase systems with fluctuations, two-phase and multi-component systems with sea-island structures, as well as scattering from fractal and polymer systems.