Leader-member exchange (LMX) is the foremost dyadic theory in the leadership literature. Whereas contemporary leadership theories such as transformational, servant, or authentic leadership theories focus on the effects of leader behaviors on employee attitudes, motivation, and team outcomes, relational leadership theory views the dyadic relationship quality between leaders and members as the key to understanding leader effects on members, teams, and organizations. This approach views trust- and respect-based relationships as the cornerstone of leadership. LMX has grown from a new theory in the 1970s to a mature area of research in 2015. Interest in this theory has increased rapidly over the past four decades, and the pace of research in this area continues to accelerate dramatically. The Oxford Handbook of Leader-Member Exchange takes stock of the literature to examine its roots, what is currently known, what research gaps may exist, and what areas are in need of the most urgent research.