Small-angle X-ray scattering (SAXS) was discovered in 1938 by A. Guinier.(1) It is now a powerful method for characterizing catalysts (particle size, surface area) and disordered materials such as gels, sols, defective alloys, porous oxides or carbons, polymers. Like diffraction, SAXS is a coherent scattering phenomenon, but instead of being produced by the interference of waves scattered by atoms ordered inside a unit cell smaller than 20 A, SAXS originates from the interference between larger blocks of uniform matter whose diameters are typically 20 to 1000 A. Then, since direct and reciprocal space are related by Fourier transforms, the normal diffraction is observed at large angles (Sin 2θ/λ = 0.1 to 1.5 A−1), SAXS is limited to a narrow cone near the origin (< 0.1 A−1). It is easily understood, from the very nature of this diffraction phenomenon, that its interpretation will be straightforward for a two-phase system, with uniform density in each phase, but much more intricate for multicomponent catalysts. In this case complementary information (electron microscopy, gas adsorption) will be indispensable for interpretation of the scattering data.