Complexity emerges from dependencies: structure governs behavior. This chapter equips designers to see and steer complexity using tools such as Design Structure Matrices (DSMs) and change-propagation analysis. We show how to expose hidden couplings, localize risk, partition architectures, and design interfaces that contain rework. Case patterns connect system architecture to emergent behaviors and socio-technical coupling, offering tactics to align product, process, and organizational architectures so systems remain evolvable, testable, and governable.