Is one circuit significantly more conservative or liberal than the others? Do circuit courts consistently avoid deciding the substance of certain appeals by concluding that the plaintiffs lack standing? Have state governments been more successful than other parties when they appeal adverse district court rulings? Do appeals courts act in a majoritarian or countermajoritarian manner with regard to elected institutions and the general public? The United States Courts of Appeals Data Base, an extensive data set of courts of appeals decisions, can address these and other questions about the circuit courts. This article describes the background, scope, and content of the database, explains how to use it, and illustrates applications to research questions of interest to the diverse law and social science community interested in courts of appeals.