Grazing X-ray reflectometry is used in order to characterize thin layer stacks, in particular periodic multilayers. The specular reflectivity depends on the thickness, the complex refractive index of each layer and on the roughness of the interfaces. By a trial and error method, the experimental reflectivity curve can be fitted with a theoretical one, and so, the parameters of the stack can be obtained. This numerical method needs usually initial guess of the kind of results. Fourier transform method allows to obtain directly the values of distances between interfaces, with a good approximation depending on the maximum angular scan of the measure. It can also reveal some particularity of the multilayer, i.e. periodic multilayered structures with more than two layers per period. As an illustration of this characterization method, some examples in XUV optical domain will be shown. This method can also be used for the characterization of many kinds of multilayer stacks, in particular semi-conductor heterostructures ones, under the condition that adjacent layers have sufficient contrast index at the wavelength of the X-ray source. (c) 2006 Elsevier B.V. All rights reserved.