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The ISA is a logical representative encoding of the basic set of distinct operations that a computer architecture may perform, and by which application programs specify the useful work to be done. At the machine level the hardware system directly interprets and executes a sequence or partially ordered set of these basic operations. This is true for all computer cores, from those few in the smallest mobile phones to potentially millions making up the world's largest supercomputers. High performance computer architecture extends structure to a hierarchy of functional elements, whether small and limited in capability or possibly entire processor cores themselves. In this chapter many different classes of structure are presented, each exploiting concurrency in its own particular way. Research in cohesion and coupling has its roots in the early 1970s as part of the development of modules. Structured design formalized the process of creating modules, recognizing that better written mo