X-ray reflectivity (XRR), a technique related to X-ray diffraction (XRD), is becoming a widely used tool for the characterization of thin-film and multilayer structures. X-ray scattering at very small diffraction angles allows characterization of the electron density profiles of thin films down to a few tens of Ǻ thick. Using a simulation of the reflectivity pattern, a highly accurate measurement of thickness, interface roughness and layer density for either crystalline or amorphous thin films and multilayers can be obtained. No prior knowledge or assumptions regarding the optical properties of the films are required, unlike optical ellipsometry