X-ray diffraction is the main tool used to obtain the atomic structure of 3-D crystals. The relatively weak interaction of the X-ray beam with matter and the resulting large penetration depth makes it insensitive to structural details in a small surface area: the surface is therefore usually neglected in structure determinations of 3-D crystals. This has changed with the development of synchrotron radiation as an X-ray source which provided new applications in X-ray crystallography. The very high intensity and angular resolution of the synchrotron beam allow the study of numerous effects which had been considered too weak to detect with laboratory X-ray sources. It has been shown, however, that with intensive X-ray sources that are available now, the structure analysis at surfaces is also possible in the laboratory.