This article identifies three distinct concepts of workplace freedom of association, and traces their influence on the law of union security devices — contractual clauses that require workers, on pain of termination, to remit fees to unions. The “social democratic” concept informed the passage of the NLRA and continues to inform social movement practice. It views workplace freedom of association as a means to the end of ensuring economic democracy, and endorses the so-called “union shop,” under which workers must contribute both to unions’ representational activities and to their political and legislative activities. The “civil libertarian” concept was predominant in Supreme Court doctrine from the Warren Court era until recently. It emphasizes individual rights of expression and political participation, and backstopped the line of cases that declared the union shop unlawful but required workers to help defray representational expenses. The “neoliberal” concept now appears ascendant. It views market behavior as a form of expressive behavior, and views compulsory payment of any fees to unions as unconstitutional compelled association. Disaggregating these concepts can enrich debates around …