Culture is the accumulated pattern of shared assumptions that a group has developed over time, making it resistant to rapid transformation despite executive mandates. Groysberg's research shows that successful culture change requires aligning formal mechanisms (structures, processes, incentives) with informal ones (rituals, stories, networks) over a minimum three-to-five-year horizon. The article identifies the four most common failure modes and provides a phased approach that maintains energy through quick wins while pursuing deep structural changes.