When X-rays strike a surface at glancing incidence they can reflect off the surface. However, if the surface is rough or covered by a film, then the X-ray reflectivity of a surface can change. XRR takes advantage of this effect by measuring the intensity of X-rays reflected from a surface as a function of angle. Thin films on a surface can give rise to oscillations of the X-ray intensity with angle. XRR can provide information on the thickness, roughness and density of thin films on a surface.