National Solid Wastes Management Association (plaintiff) filed suit in federal court to challenge an Illinois law that imposed training, examination, experience and licensing requirements upon employees at hazardous waste facilities and operators of hazardous waste management equipment. The Association asserted that the state statutes were pre-empted by federal Occupational Safety and Health Act standards and violated the Commerce Clause of the United States Constitution. The district court found that the Illinois laws were not pre-empted by OSHA regulations because they served the purpose of protecting public safety in addition to the OSHA purpose of promoting workplace safety. The district court invalidated a portion of the laws that required mandatory training to take place in Illinois on grounds that it did not promote the purpose of protecting public safety. The court of appeals affirmed in part and reversed in part, and the state of Illinois (defendant) petitioned the United States Supreme Court for review.