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Although there has been an explosion of empirical legal scholarship about the federal judiciary, with a particular focus on judicial ideology, the question remains: how do we know what the ideology of a judge actually is? For federal courts below the U.S. Supreme Court, legal academics and political scientists have offered only crude proxies to identify the ideologies of judges. This Article attempts to cure this deficiency in empirical research about the federal courts by introducing a new technique for measuring the ideology of judges based upon judicial behavior in the U.S. courts of appeals. This study measures ideology, not by subjectively coding the ideological direction of case outcomes, but by determining the degree to which federal appellate judges agree and disagree with their liberal and conservative colleagues at both the appellate and district court levels. Further, through regression analysis, several important findings related to the Ideology Scores emerge. First, the Ideology Scores in this Article offer substantial improvements in predicting civil rights case outcomes over the leading measures of ideology. Second, there were very different levels and heterogeneity of ideology among the judges on the studied circuits. Third, the data did not support the conventional wisdom that Presidents Ronald Reagan and George W. Bush appointed uniquely ideological judges. Fourth, in general judges appointed by Republican presidents were more ideological than those appointed by Democratic presidents. Fifth, attendance at a higher-ranked law school was strongly correlated with liberalism on the bench for appointments of both Republican and Democratic presidents. Sixth, prior work experience in the government (outside the judiciary) indicated liberal judicial voting.