Abstract: This is the first chapter of Are Judges Political? An Empirical Analysis of the Federal Judiciary. The book reports the results of a large-scale empirical study of judicial behavior on the federal appellate courts. There are three key findings. First, the political party of the president who appointed the judge matters - judges appointed by Republican presidents vote differently from Democratic appointees across a range of important cases. These are party effects. Second, even in hard cases, the law constrains judicial behavior - Republican and Democratic appointees tend to agree more often than they disagree. So although party effects exist, they do not fully determine judicial behavior. Finally, group dynamics are critical - a Democratic appointee sitting with two Republicans votes differently from a Democratic appointee sitting with two Democrats, and similar patterns hold for Republican appointees. We identify several group dynamics that affect the voting behavior of judges sitting on panels, including the collegial concurrence, group polarization, and the whistleblower effect. Moreover, panel effects are frequently as large as party effects. In other words, the party that appointed a judge is often no more predictive of that judge’s vote in a given case than the party that appointed the two other judges sitting on the same panel in that case. While this study cannot provide conclusive answers to contested questions regarding judicial behavior and the design of the federal courts, we hope that the data reported here can inform the resolution of those questions.