Thin solid films and surfaces are characterized by FTIR-spectroscopy, based on reflectance and transmittance measurements, in particular with polarized light at oblique incidence. Thus two independent data sets for s- and ppolarization are available. At p-polarization additional absorption lines at the zeros of the dielectric function are observed (Berreman-effect). The interpretation of the measured spectra is carried out by the following procedure: the observed reflectance and transmittance are simulated by spectra, calculated from a model by an optimum fit of the model parameters. As a result the specimens are characterized in terms of vibronic resonances, contribution of free electrons and holes, thicknesses of the various layers in a stack, and of profiles of chemical composition. Thicknesses and roughness of very thin layers are determined from measurements of the reflectance of X-rays at grazing incidence. The interpretation is also based on model simulations. In detail the following examples are discusses: thin Ag-films deposited on glass, oxide films on semiconductors and metals, depth depending profiles ofextrinsic atoms due to diffusion or ion implantation, and semiconducting multilayer structures. The reliability of the non-destructive IR-method is compared with other relevant analytic methods as SIMS, RBS, and AES.