The Arizona state legislature passed the Legal Arizona Workers Act (LAWA), which permitted the state to suspend licenses of, and sue, employers who hired unauthorized aliens. The law also required that Arizona employers use a federal electronic verification system to confirm that all workers were authorized to be employed in the United States. The Chamber of Commerce of the United States and others (collectively Plaintiffs) filed suit in federal district court against those charged with administering the law, including county attorneys, the Governor of Arizona, the Arizona Attorney General, and others (collectively Defendants), alleging that the Arizona law was preempted by the federal Immigration Reform and Control Act (IRCA). The district court held that the Arizona law was not preempted and Plaintiffs appeal. The court of appeals affirmed, and held that Arizona’s law fell within the regulatory scheme of the IRCA and was thus not preempted. The U.S. Supreme Court granted certiorari to review.