System architects face the formidable task of purposefully shaping an evolving space of complex designs. Their task s further complicated when they lack full control of the design process, and therefore must anticipate the behavior of other stakeholders, including the designers of component products and competing systems. This paper presents a conceptual tool called a design structure network (DSN) to help architects and design scientists reason effectively about these situations. A DSN is a graphical representation of a system’s design space. DSNs improve on existing representation schemes by providing a compact and intuitive way to express design options—the ability to replace all or part of one design with another. Design options, in turn, are the building blocks of architectural strategy—the practice of designing systems with an awareness that the fortunes of other designers are intertwined with one’s own. I illustrate the informal use of design structure networks with an example based on Apple’s decision to adopt the Intel processor architecture. I also show how DSNs can serve as a formal foundation for economic models of architectural strategy, which I call system design games.