Memory is the architectural construct through which a unit preserves the interpretive structure of its past, enabling continuity, accountability, and learning over time. It records not only what happened, but why it happened and how it informs future action—making decisions, behaviors, and changes intelligible across agents and contexts. Without Memory, systems lose coherence: actions cannot be explained, outcomes cannot be traced, and adaptation becomes reactive rather than intentional. In Purpose Driven Design, Memory is distinct from Storage—it does not retain artifacts or observe the present, but preserves the structured history that allows systems to understand themselves. This chapter establishes Memory as the mechanism that stabilizes identity, supports governance, and enables evolution in sociotechnical systems..