A new organizational form is emerging in companies that run on knowledge: the community of practice. And for this expanding universe of companies, communities of practice promise to radically galvanize knowledge sharing, learning, and change. A community of practice is a group of people informally bound together by shared expertise and passion for a joint enterprise. People in companies form them for a variety of reasons–to maintain connections with peers when the company reorganizes; to respond to external changes such as the rise of e-commerce; or to meet new challenges when the company changes strategy. Regardless of the circumstances that give rise to communities of practice, their members inevitably share knowledge in free-flowing, creative ways that foster new approaches to problems. Over the past five years, the authors have seen communities of practice improve performance at companies as diverse as an international bank, a major car manufacturer, and a U.S. government agency. Communities of practice can drive strategy, generate new lines of business, solve problems, promote the spread of best practices, develop people’s skills, and help companies recruit and retain talent. The paradox of such communities is that although they are self-organizing and thus resistant to supervision and interference, they do require specific managerial efforts to develop them and integrate them into an organization. Only then can they be fully leveraged. The authors explain the steps managers need to take in order to get communities going–and to sustain them so they can become a central part of their companies’ success.