General Nutrition Companies, Inc. (GNC) (defendant) operated retail stores that sold nutritional supplements. Some of these supplements contained androstenediol, which was a Schedule III controlled substance under California law. Selling or possessing Schedule III substances without a prescription was illegal. Diego Martinez (plaintiff) bought supplements from GNC containing androstenediol. Martinez sued GNC, alleging violations of California’s Unfair Competition Law (UCL) and Consumer Legal Remedies Act (CLRA). Martinez claimed that he would not have purchased these supplements if he had known they were illegal. Martinez moved to certify a class of all people who had purchased these supplements between 2000 and 2004. GNC opposed the motion, arguing that each plaintiff would need to individually establish causation and injury. The trial court agreed and refused to certify a class. Martinez appealed to the California Court of Appeal.