Illinois Brick Company (Illinois Brick) (defendant) was a manufacturer of concrete bricks operating in the Chicago area. Illinois Brick sold bricks to masonry contractors, who then submitted bids to general contractors for specific portions of larger construction projects. The State of Illinois (plaintiff) awarded construction projects to general contractors who accepted bids from masonry contractors using bricks purchased from Illinois Brick. As a result, Illinois was an indirect purchaser of Illinois Brick’s products. Illinois believed that an unlawful price-fixing conspiracy by Illinois Brick had caused Illinois to be overcharged on construction projects by a significant amount. Illinois brought a lawsuit, seeking treble damages under § 4 of the Clayton Act. Illinois Brick moved for summary judgment on the theory that indirect purchasers were not eligible to sue under § 4 for overcharges. The court of appeals rejected the motion, and Illinois Brick appealed.