Line Profile Analysis is not difficult, but it is complicated. Because it is not difficult it is tempting to think that it is simple and easy to apply, whereas one should be careful because it is complicated matter. Therefore experts in the field exist and it is to be regretted sincerely that their expertise and experience is not yet translated into algorithms - which is a complicated and time consuming task if possible at all. Diffraction-line broadening is determined by (a) contributions from the instrument ("instrumental aberrations"), (b) by the spectral distribution of the radiation used, and (c) the microstructure of the specimen, i.e. small crystallite size and lattice strains caused by lattice defects, e.g. dislocations, small inclusions or precipitates, stacking and twin faults. Although it is known for half a century that some of these microstructural phenomena do not only influence diffraction-line breadth and shape, but also diffraction-line positions, it is only rather recent that their influence is taken into account more commonly in structure determination, and in particular in structure determination from powder diffraction patterns. Examples of such causes of peak shifts are stacking and twin faults as well as macrostresses (residual stresses). Just reviewing the literature on the analysis of diffraction-line broadening does not lead to better understanding or to new insights and it does not help non-experts to understand and apply these methods. Therefore it will be tried to reconstruct the Evolutionary Tree of Diffraction-Line Broadening Analysis Methods with the main intention to order its present scientific basics and possibilities along with its methods / techniques. In addition some examples will be given of the relations of line-broadening analyses with other branches of diffraction analysis and with fields of application such as materials science, crystal structure determination, microstructure determination, residual stress determination, and texture determination. In today’s practice simple procedures that are easy to apply (e.g. Williamson Hall plots) are used widely and frequently to determine whether the observed broadening of diffraction lines is generated by the presence of finite crystallite dimensions and/or by lattice strains on a nanometer scale. In this respect they are an obligatory "instrument" and truely helpful. But it is amazing that they are applied often without justification for the (semi)quantitative analysis of crystallite sizes and their distribution and the various types of lattice defect that cause lattice strain (size-strain analysis). The increase in power of PC's makes the use of more dedicated and complicated methods of diffraction-line broadening analysis less prohibitive, but the very detailed, complicated, and often unknown crystal-structure dependence of the lattice defects prevent the writing of straightforward, generally applicable computer programs: software becomes the bottleneck. Several examples will be given of cases of misuse as well as correct use and of probable future practical applications. The main conclusion will be that there is a need for a joint, international effort: (i) to identify the implications of diffraction-line profile analysis for other diffraction analyses like crystal-structure determination, in particular from powder data, (ii) to develop algorithms for these implications, and (iii) implement these algorithms in software.

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