It is well known that there are close correlations between structural and magneticproperties of materials and certainly this holds also for nanostructured epitaxialfilms. So, the knowledge of the crystallography of a certain structure - i.e. the knowledge of the precise coordinates of the atoms involved - is essential for a quantitative understanding of a system's magnetic properties. Unfortunately, realspace methods such as scanning tunneling microscopy provide crystallographic dataof only rather limited accuracy and - even worse - only of the top layer. Thefull structure of a film can only be resolved by techniques applying surface penetratingprobes, for example, x-rays or electrons (or both).The present talk illuminates the power and limitations of using electrons or, moreprecisely, low-energy electron diffraction (LEED) in its quantitative version (applyingtensor LEED for the intensity analysis). The talk concentrates on metallic epitaxialfilms of nickel, cobalt and iron on low index copper surfaces as well as iron on a reconstructediridium surface. It will be shown that in favourable, i.e. structurally simple cases, quantitativeLEED can resolve atomic positions with an accuracy of the order of 0.01Å, as well as the chemicalnature of the atom under consideration. However, the accuracy reduces with increasing structural complexity. This is because of both correlations between parameters and, quite often, some lack of the scientist's imagination in considering all relevant parameters.Complexity can hold even in simple cases when different structural domains exist, with each ofthem to be analysed, but only the sum of the intensities is available for the fit. On theother hand, the method can be extremely sensitive to a parameter (for example, vertical layer spacings) or to a certain structural arrangement. An example is the stacking of layers during growth, so that different sequences of, for example, fcc and hcp stackingcan be clearly differentiated.Complex crystallographic structures are unavoidable when there is competition betweenpseudomorphic growth and a film's tendency to assume its native structure. Then, substantialdistortions can develop within the film as, for example, in the case of iron on reconstructediridium, where the substrate's structure influences the film growth in an unusual way. The power ofLEED is demonstrated for these scenarios also.