Few institutions in the world are credited with initiating and confounding political change on the scale of the United States Supreme Court. The Court is uniquely positioned to enhance or inhibit political reform, enshrine or dismantle social inequalities, and expand or suppress individual rights. Yet despite claims of victory from judicial activists and complaints of undemocratic lawmaking from the Court’s critics, numerous studies of the Court assert that it wields little real power. This book examines the nature of Supreme Court power by identifying conditions under which the Court is successful at altering the behavior of state and private actors. Employing a series of longitudinal studies that use quantitative measures of behavior outcomes across a wide range of issue areas, it develops and supports a new theory of Supreme Court power. Matthew E. K. Hall finds that the Court tends to exercise power successfully when lower courts can directly implement its rulings; however, when the Court must rely on non-court actors to implement its decisions, its success depends on the popularity of those decisions. Overall, this theory depicts the Court as a powerful institution, capable of exerting significant influence over social change.