Culture is the architectural construct through which a unit shapes behavior, distributes interpretation, and enables coherent action without constant rule enforcement. It operates as an implicit decision infrastructure—guiding how agents respond to ambiguity, prioritize outcomes, and align with Purpose in the absence of explicit instruction. Without Culture, systems rely excessively on rules and processes, becoming brittle, slow, or misaligned under changing conditions. In Purpose Driven Design, Culture is not an atmospheric by-product but a structurally embedded layer—expressed through interaction patterns, feedback loops, incentives, and everyday practices. This chapter establishes Culture as the mechanism that enables adaptive coherence, sustaining alignment, trust, and responsiveness across agents, units, and environments.