The diffraction of low energy electrons is mostly used either for qualitative information (e.g. existence of superstructures) or for determination of atomic positions within unit mesh via intensity evaluation assuming a strictly periodic surface. A lot of additional information is available as soon as, besides existence and intensity of a spot, its profile on the screen (in general, profile in k-space) is used. In this way all kinds of deviation — from simple periodicity, periodic or non-periodic — may be derived quantitatively and qualitatively without much computation. For a qualitative evaluation, a set of basic structural elements and their representation in reciprocal space is discussed, so that a lot of imperfections like steps, islands, domains, point defects and some elements of their arrangement (like random or regular) are easily identified just by visual inspection of the LEED pattern. For quantitative eveluations, the limitations due to the instrument have to be considered. They are described by the average quantity "transfer width", the maximum distance of surface atoms for a coherent diffraction. New instruments show transfer widths of more than 200 nm, so that the maximum resolvable defect distance is a multiple of that length. For quantitative evaluation of non-constant island sizes, terrace widths and so on, assumption or information on distributions (random or correlated) is needed. Examples of defects from 0 to 3 dimensions are discussed: point defects due to static or thermal disorder, linear defects as borderlines of islands and domains or as steps between terraces, planar defects as facets and bulk defects due to mosaic structure or strain. A wide field is open for further use of spot profile analysis (SPA-LEED).