Light scattering measurements can be used both for quality control and as a tool for characterization of thin film properties. Thin films contribute scattering from surface irregularities at interfaces and from inhomogenities in the film structure. For multilayer stacks correlation effects may also be present. The most refined scattering theories invoking both surface roughness and inhomogeneous dielectric permittivity show good agreement for a limi-ted number of experimental cases. Grain sizes and shapes in the film, are not treated in this theory but these become increasingly important at the shorter wavelengths, i.e. in UV and EUV. In general, incident polarized light and angle-resolved detection of the scattered light is a good way to get a more fundamental understanding of the thin film scattering process. By extending the scatter measurements into the extreme ultraviolet, more information about the sub-micron irregularities can be gained.