This chapter explains the need for system architecture, both to understand existing complex systems and to design and implement new systems. “System architecture” is a term borrowed from the building industry where an architect specifies the look and feel of a building, including its intended usage and purpose. The same architecture can then be used again and again, although some minor modifications within the constraints of the architecture may be required for individual buildings. This chapter reviews different designs of enterprise architecture and discusses the key concepts behind these architectures and how they can be applied.