How much knowledge should leaders have of their organization's core business? This is an important question but not one that has been addressed in the management literature. In a new 'theory of expert leadership' (TEL), this paper blends conceptual work with recent empirical evidence. It suggests that organizations perform more effectively when led by individuals who have a deep understanding of the core business of their organization. Being a capable general manager is not sufficient. Expert leaders are those with (1) inherent knowledge, acquired through technical expertise combined with high ability in the core-business activity; (2) industry experience, which stems from time and practice within the core-business industry; and (3) leadership capabilities, which include management skills and a leader's innate characteristics. This paper criticizes the rise of the professional manager and generalist CEO. It argues that expert leaders improve organizational performance through knowledge-based strategy, by acting as a standard bearer, by creating the right environment for core workers, and, finally, by adopting the long view. The paper concludes by identifying the potential boundaries of TEL.