The science of physical metallurgy is a very broad topic that includes the structure of metals. This article introduces the general structural features of metals. It provides background, general references, and some connections among the subject matter explored more fully in the specialized articles included in the other parts of this section. The article includes information on the origins of structures, and single-phase and multiphase microstructures. It discusses some important topics, such as grain structure and substructure, that are not covered systematically and comprehensively in the other articles. Finally, the article also brings in the scale of structural features and the concept of hierarchical relations among them.