This paper reviews the main idea underlying the use of network graph theory for analysis or for design of physical engineered systems. A physical engineered system is a system built from physical components, as compared with a system built only from symbols or software. The term includes structures, mechanisms, electric circuits and more. Different engineered systems may be represented as the same graph, or as graphs that show a known mathematical relationship between them. We then have a single mathematical representation that is applicable to more than one engineered system. The properties of the graph, as known from graph theory, are applicable to all the engineered systems in domains that match that graph. The graph can be regarded as a generalized representation suitable for various engineered systems. Engineering theory is commonly divided into domains, solid mechanics, mechanisms, fluid mechanics, heat transfer, and more. When dealing with engineered systems using the language and mathematical formality of graph theory such divisions become unnecessary. Network graph theory can apply similar or even identical theory to many engineering domains.