Structure is the architectural construct through which a unit assigns responsibility, configures agents, and enables accountable action. It defines who or what acts, how roles are bounded, and how authority, escalation, and coordination are organized across human, artificial, and hybrid participants. Without Structure, Purpose cannot be enacted, Function cannot be operated reliably, and Process cannot be sustained—leaving systems vulnerable to ambiguity, fragmentation, and failure. In Purpose Driven Design, Structure is not an organizational chart but a dynamic configuration of roles and relationships that supports delegation, embeds accountability, and maintains coherence under changing conditions. This chapter establishes Structure as the coordinating framework that transforms intent into assignable action and enables resilient, composable sociotechnical systems.