In Giglio v. United States, the Supreme Court ruled that prosecutors are required to disclose any deals or promises made with witnesses in exchange for their testimony. The case revolved around John Giglio, who was convicted of forgery based largely on the testimony of a co-conspirator named Taliento. During trial, it emerged that while Taliento had initially been indicted alongside Giglio, his charges were dropped after he agreed to testify against him - a fact not disclosed by prosecution during trial. On appeal, the Supreme Court overturned Giglio's conviction stating that failure to inform the jury about an agreement between prosecutor and witness violated due process rights as defined under Brady v Maryland (1963). This decision expanded upon Brady ruling which mandated disclosure of exculpatory evidence only; post-Giglio prosecutors now also needed to reveal impeachment evidence i.e., information affecting credibility of government’s witnesses.

In the dissenting opinion for Giglio v. United States, Justice William H. Rehnquist argued that the majority's decision to overturn Giglio's conviction based on a failure of the prosecution to disclose an alleged promise of immunity made to its key witness was misguided and could potentially lead to unnecessary retrials in other cases where such promises were not disclosed but had no significant impact on the outcome. He contended that it should be up to trial judges, rather than appellate courts, to determine whether or not such undisclosed promises are material enough in each specific case context - i.e., if they would have likely affected jury verdicts - before deciding whether new trials are warranted. The justice also expressed concern about how this ruling might affect future prosecutorial conduct and decisions regarding what information must be shared with defense attorneys prior their clients' trials.