After a brief discussion of the theoretical basis of substructures for statics and dynamics, the handling techniques are described, including file management, modification utilities, and nested heirarchies. Examples of recent applications illustrate the economies that can be achieved by substructuring. Substructures are especially economical in repetitive analyses, such as nonlinear static and dynamic analyses in which major portions of the structure exhibit linear behavior, or in design studies where only a portion of the model is to be modified. Substructuring is not generally economical in single load case static analyses but it may be necessary in some cases of very complicated models, in order to analyze them within available computer resources. Some of the tradeoffs and pitfalls are also presented. The principles of substructuring are extended to include transient thermal analyses, which may be coupled to substructured stress analyses. The role of substructures as an intergroup communication tool is outlined.