Police unions raise issues of great importance for political scientists. Yet, the field has neglected them. This essay argues that political scientists should see police unions as important interest groups, empowered by state collective bargaining laws, that are important players in local politics and shapers of the criminal justice system in America. The organizational properties that make police unions important interest groups are described. The important political questions that arise once we consider police union as interest groups are examined. The existing research on police unions—especially their impact on government costs and police behavior—is detailed. Ultimately, the study of collective bargaining in law enforcement and police union political activity in local politics has been overlooked because it lies at the intersection of three different streams of scholarly research—interest groups, local politics, and policing.