In the very beginning of materials �science�, when men began to produce artificial materials, it was the time of trial and error, of pure empiricism. Today, we are a little closer to the realization of the old dream of designing any material with given properties owing to our improved understanding of the relationships between chemical composition, crystal structure and material properties. Though only a very few commercially and technologically important materials consist of metallic elements in their pure form (Si, Ge, Cu, Au, Ag, Pd, etc.), their crystal structures are of more than academic interest. Thus, to give an example, the crystal structure of a pure metal remains unchanged in the case of a solid solution, when one or several other components are added to tune the properties of a material. This technique has been used since time immemorial by alloying gold with copper or silver, for instance, to make jewelry or coins more resistant to wear. Especially the close packed structures and their derivatives, which are typical for pure metals, are also characteristic for numerous materials consisting of multi-component solid solutions or intermetallic alloys. Another reason for the study of �simple� element structures is that they are extremely helpful for the development and improvement of methods to understand why a given phase is adopting a particular crystal structure under certain conditions (temperature, pressure, etc.). The aim is, of course, to learn to predict the crystal structure of any given chemical compound under any ambient conditions and to model its possible phase transformations.