The Bragg peaks are seen to broaden significantly resulting in an overlap between neighboring peaks like the group of 221, 310 and 311. In this example the size distributions were determined by transmission electron microscopy(lead sheet) (PEI; see Fig. 3.7(a) and (b)), and were found to be rather monodisperse, i.e. the standard devia- tion of the size distribution function amounted to only 7% of the average size. Rather often, polycrystalline samples exhibit pronounced variations in crystallite size with grains including dislocation networks, stacking faults, twin planes and other lattice faults. These distortions can severely complicate the interpretation of the peak broadening. In such complex problems like the microstructure of solids the interpretation of the results strongly depends on underlying model assumptions. A set of formulas is presented in the following, which relate microstructure parameters and line profile parameters. When applying these formulas, it should always be kept in mind that they are only valid for some restricting assumptions on the microstructure, i.e. within the validity range of the underlying model. The models should be subjected to mistrust and results derived from them should only be given with simultaneously indicating the basic assumptions. This section is con- cemed with models that neglect any broadening due to microstrain and other lat- tice distortions, but solely account for small crystallite size. The presentation starts by assuming a polycrystalline sample to be comprised of crystallites having all the same shape and dimension. Although this is a purely hypothetical case, it allows or the introduction of the relevant quantities and the derivation of the Scherrer equation. The discussion is restricted to the case of cubic materials.