A. van der Lee, Institut Européen des Membranes An outline is given for the use of X-ray grazing incidence reflectometry for studying porous thin films. X-ray reflectivity is a technique which is close to X-ray diffraction, but which can be used for amorphous thin films as well. The technique can be used to extract the vertical density profile of a thin film whenever this film is homogeneous parallel to the surface. The basics of the technique are described, and its relation with diffraction and optics, in particular ellipsometry, via the extended Bragg's law and Snell's law. A virtual reflectivity experiment is presented in order to explain the main features of a typical X-ray reflectivity curve and the information that can be extracted from it. Practical issues of the data collection, equipment and data treatment are discussed. The weak and strong points of the technique are given in comparison with other thin film characterization techniques. A number of experimental examples is presented in order to illustrate the use of the technique. In particular it is explained how X-ray reflectometry can be used for the determination of the pore size distribution in a thin film.