This chapter discusses methods of investigation of structural defects at surfaces. For a study of defects at clean surfaces all investigations have to be done in a condition, where the surfaces stay clean. Therefore in most cases ultra high vacuum (pressure less than some 10-10 mbar) is required. The same vacuum has to be used first to produce the surface to be studied by cleaning, annealing and modifying whatever is wanted, and second to study the surface structure with respect to the perfect, periodic arrangement and to deviations from periodicity, that are the defects. In some cases the defects may be sealed in by decoration or by a protecting film, so that the sample may be transferred out of the ultra high vacuum into a different system for investigation. Sometimes just the protecting film is lifted off and used for defect study. All methods (in-situ and ex-situ) may produce defects themselves; a lot of defect studies is therefore devoted to that problem. The ideal experimental method should show a UHV compatibility, should have surface sensitivity and atomic resolution or at least sensitivity for atomic defects and should not disturb or modify the surface. Even if only part of the conditions are met, the method may be very useful for surface defect analysis. The various experimental techniques used for the investigation are microscopy, diffraction, ion scattering, and other indirect methods.