In the common law tradition, legal decisions are supposed to be grounded in both statute and precedent, with legal training guiding practitioners on the most important and relevant touchstones. But actors in the legal system are also human, with the failings and foibles seen throughout society. This may lead them to take methodological shortcuts, even to relying on unknown internet users for determinations of a legal source’s relevance. In this chapter, we investigate the influence on legal judgments of a pervasive, but unauthoritative source of legal knowledge: Wikipedia. Using the first randomized field experiment ever undertaken in this area – the gold standard for identifying causal effects – we show that Wikipedia shapes judicial behavior. Wikipedia articles on decided cases, written by law students, guide both the decisions that judges cite as precedents and the textual content of their written opinions. Collectively, our study provides clear empirical evidence of a new form of influence on judges’ application of the law – easily accessible, user-generated online content.