This chapter discusses diffraction methods for studying structural defects at surfaces. The diffraction methods are more indirect methods. Diffraction pattern is a Fourier transform of the surface and has to be discussed with the help of reciprocal space. Whereas microscopies are to be preferred for qualitative analysis, the diffraction methods are the necessary counterpart for quantitative analysis. The diffraction pattern of a crystalline surface provides maxima at spots corresponding to the periodicity of the surface. Spot position and integral intensity are used to derive the atom positions in the unit mesh of a periodic structure. The techniques have in common the atomic resolution and the UHV compatibility, so that all investigations are done in-situ, frequently in real time during modifications. Electrons are used with low energy and (nearly) normal incidence (Low Energy Electron Diffraction LEED, SPA-LEED). The higher the electron energy the more grazing angles for the incident and the exiting beam have to be chosen (e.g. E = 20 keV, 1º grazing incidence, Reflection High Energy Electron Diffraction RHEED). Using grazing incidence and a high intensity source (synchrotron radiation, preferably a wiggler or undulator station) surface sensitivity and sufficient count rates for defect analysis are available, if the scattering vectors avoid the 3D Bragg conditions.