LEED has found widespread application in surface science, since the LEED experiment can be performed in a small laboratory and LEED systems are commercially available. A main advantage compared to surface X-ray diffraction is that on the LEED screen most of the 2-D diffraction pattern is visible, thus allowing a quick and comprehensive overview of the symmetry and to some extent about the degree of ordering of the surface under examination. A LEED system is therefore included in most UHV chambers to control the quality of the surface preparation for a wide range of surface studies. A qualitative interpretation of the diffraction pattern is the most common use of LEED: it allows the identification of the surface unit cell, the estimation of the degree of ordering and the identification of different surface phases in adsorption systems (and thereby often a check on adsorbate coverage). The diffraction pattern thus reflects the translational symmetry and the crystalline order of the surface.