Diffraction studies of nonstoichiometric compounds have revealed various diffusion scattering effects caused by formation of a short-range order related to substitutions of atoms and vacancies (or different-sort atoms) or to atomic displacements. For nonstoichiometric compounds, we consider experimental results on short-range order that are obtained from diffuse neutron and X-ray scattering and electron diffraction. The occurrence of diffuse intensity maxima in diffraction patterns is shown to result from a redistribution of nonmetallic atoms and structural vacancies in disordered nonstoichiometric carbides, nitrides, and oxides or of mutually substitutable atoms in solid solutions of nonstoichiometric compounds at the stage preceding the formation of a long-range order. Applications of the transition-state cluster model for the description of the topology of the diffuse scattering intensity in nonstoichiometric compounds with the substitutional short-range order are discussed. Flat extended diffusive scattering regions obtained in diffraction patterns for ordered phases, not passing through structural sites of the reciprocal lattice, are shown to occur due to atomic displacement waves.