Marcos Sanchez (defendant) was charged with gun, drug, and gang-related offenses. To prove that Sanchez was affiliated with a street gang, the State of California (plaintiff) called police detective David Stow to testify at trial as a gang expert. Stow told the jury about the contents of various notices and identification cards that were kept in a state gang database. The notices contained statements from other officers about their interactions with Sanchez and statements from Sanchez himself about his association with the gang. Sanchez objected to Stow’s testimony, arguing that Stow’s discussion of the out-of-court statements violated both the evidentiary rules prohibiting hearsay and the Confrontation Clause of the Sixth Amendment. The state argued that the statements were not hearsay because they were offered only to show the basis for Stow’s expert opinion and not for the truth of the matter contained in the statements. The trial court allowed the testimony. The jury convicted Sanchez, and he received a penalty enhancement based on his gang activity. Sanchez appealed his conviction. The appellate court reversed the conviction on one offense but otherwise affirmed, and Sanchez appealed to the California Supreme Court.