The Supreme Court’s decision last term in Harris v. Quinn is its latest, and surely not its last, of dozens of encounters since the 1940s with the issue of mandatory union fees – fees that individuals may be required to pay as a condition of employment to the union that represents them in collective bargaining. Along the way to striking down the “agency fee” in Harris on First Amendment grounds, the Court proclaimed the current agency fee regime, blessed many times by the Court itself, to be an “anomaly.” This Article asks whether and how labor unions are themselves “anomalies” in our legal system, particularly in their constitutional entitlements. Its answer is a qualified and complicated “yes.” A solution to the agency fee problem, and to other constitutional puzzles in labor law, lies in a deeper understanding of how and why unions are distinctive in our legal system – distinctive even among private entities with public regulatory functions, which have frequently provoked constitutional controversy. To begin with, unions are voluntary associations of workers with constitutional entitlements to freedom of expression and associational autonomy. On the other hand, unions are also regulatory actors subject to a constellation of sui generis rights, powers, restrictions, and duties under U.S. labor law – a quid pro quo by which the labor laws both constrain and empower unions. Whatever the wisdom of the particular quid pro quo embodied in the labor laws – and it has been criticized from both ends of the political spectrum – it provides an essential context for the adjudication of constitutional claims by and against unions. This Article calls for reframing those constitutional claims to include both the quid and the quo of labor law – not only the alleged burdens that the labor laws impose on unions or individuals, but also any logically linked benefits or powers it confers on the claimant. The proposed analysis recasts not only the agency fee controversy and the related puzzle posed by state right-to-work laws, but also recurring challenges to the constitutionality of restrictions on union speech and recent efforts to bring “worker centers” under the umbrella of labor law. At the same time it offers clues to the future of unions and labor law if the Court continues down the path foreshadowed in Harris.