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Multilayers, i.e., a sequence of layers of different semiconductor materials in epitaxial relationship, are probably the prototype of an "artificial" semiconductor material with properties that cannot be reached in "simple" alloys of semiconductors. they have been the topic of research since many decades. Beside growth-related issues, they are still a very important class of structures. Two prominent applications are for cascade lasers structures, and for thermoelectric devices.
 
         We analyze the structure of multilayers using mainly x-ray diffraction and x-ray reflectivity methods. Issues to be addressed are the thickness and chemical composition of layers, which can in principle be measured with both approaches, as well as the interface quality in terms of interdiffusion, roughness, and the correlation of roughness from one interface to the next. The latter is the domain of x-ray reflectivity, and a variant named GISAXS (grazing incidence small angle x-ray scattering), allowing to determine not only the r.m.s. roughness, but also the frequency spectrum of the interface roughness, as well as lateral and vertical correlations properties.