In the 1978 case of Monell v. Department of Social Services, the Court had held that municipalities could be sued under § 1983 only for actions taken “pursuant to official municipal policy of some nature.”  However, the Court recognized that the phrase "official policy" provided a rough sketch rather than a detailed map of the boundaries of municipal liability and left important line-drawing questions unresolved.  For example, Monell provided no standards for determining whether a particular city official was a policy maker whose actions could subject a city to liability.  Justice O’Connor’s plurality opinion in City of St. Louis v. Praprotnik established three important rules for answering that question:  

First, the question of who has policy making authority is a question of state law rather than federal law, and is a question of law to be decided by the court rather than a question of fact to be decided by a jury.

Second, the question is to be decided solely by reference to state statutes, local ordinances, and formal acts delegating policy making authority unless the plaintiff is able to prove an informal practice so  permanent and widespread that it qualifies as a municipal custom. 

Third, an unconstitutional action by an official whose decisions are subject to review cannot be treated as a final policy unless the reviewing body approves both the decision and its unconstitutional basis. 

Justice Brennan's concurring opinion, disagreed on each of these points arguing that, while formal state and law was a starting point, juries should decide what level of authority was actually exercised by the deciding officials and what level of review was actually exercised by any reviewing body.  He agreed, however, that the case should be reversed and remanded to the Court of Appeals for further proceedings. 

Although Justice O'Connor's plurality opinion was unable to muster a fifth vote in Praprotnik itself, her position was explicitly endorsed by a five-justice majority a year later in Jett v. Dallas Independent School District.