Ever since the late 1960s, many lower federal courts have interpreted the Federal Rules of Civil Procedure to give outsiders broad rights to become parties to pending lawsuits. Intervention of this sort affects the dynamics of a lot of cases, including many of the highest-profile cases that the federal courts hear. Yet it raises fundamental questions about the structure of litigation: should status as a party be limited to people who have legal claims or defenses, or do the Federal Rules of Civil Procedure invite intervention by everyone who will feel the practical effects of a judgment? For the last half century, many federal judges and law professors have pushed for expansive understandings of the right to intervene. That impulse is consistent with the “interest representation” model of litigation, which analogizes judicial decisionmaking to other types of policymaking and touts the benefits of broad participation. According to this Article, however, the Federal Rules of Civil Procedure instead reflect a more traditional view of litigation, under which the parties to a case need to be proper parties to a claim for relief.