Massachusetts (plaintiff) enacted a mandated-benefit statute that required insurers to include minimum mental-health-care benefits for its residents. Metropolitan Life Insurance Company and Travelers Insurance Company (defendants) provided group-health policies for employers and unions with workers who lived in Massachusetts. The Massachusetts attorney general sued the insurers to enforce compliance with the statute. The insurers countered that the Employee Retirement Income Security Act of 1974 (ERISA) and the National Labor Relations Act (NLRA) preempted the mandated-benefit law. The state courts found neither federal statute preempted the Massachusetts law, and the insurers appealed. The Supreme Court granted review.