Change is hard, especially in a large organization. Yet some leaders succeed--often spectacularly--at transforming their workplaces. what makes them able to exert this sort of influence when the vast majority can't? The authors tracked 68 change initiatives in the UK's National Health Service, an organization whose size, complexity, and tradition can make reform difficult. They discovered several predictors of change agents' success--all of which emphasize the importance of networks of personal relationships: Change agents who were central in the organization's informal network had a clear advantage, regardless of their position in the formal hierarchy. People who bridged disconnected groups or individuals were more effective at implementing dramatic reforms. The resisters in their networks did not necessarily know one another and so were unlikely to form a coalition. Change agents with cohesive networks, in which all individuals were connected, were better at instituting minor changes. Their contacts rallied around the initiative and helped convince others of its importance. Being close to people who were ambivalent about a change was always beneficial. In the end, fence-sitters were reluctant to disappoint a friend. But close relationships with resisters were a double-edged sword: Such ties helped push through minor initiatives but were a hindrance when attempting major change.