280
Opinion of the Court
cause "the Stoltzfus materials would have provided little or no help . . . in either the guilt or sentencing phases of the trial." Id., at 425. With respect to guilt, the court noted that Stoltzfus' testimony was not relevant to petitioner's argument that he was only guilty of first-degree murder rather than capital murder because Henderson, rather than he, actually killed Whitlock. With respect to sentencing, the court concluded that her testimony "was of no import" because the findings of future dangerousness and vileness rested on other evidence. Finally, the court noted that even if it could get beyond the procedural default, the Brady claim would fail on the merits because of the absence of prejudice. App. 425, n. 11. The Court of Appeals, therefore, reversed the District Court's judgment and remanded the case with instructions to dismiss the petition.
II
The first question that our order granting certiorari directed the parties to address is whether the Commonwealth violated the Brady rule. We begin our analysis by identifying the essential components of a Brady violation.
In Brady, this Court held "that the suppression by the prosecution of evidence favorable to an accused upon request violates due process where the evidence is material either to guilt or to punishment, irrespective of the good faith or bad faith of the prosecution." 373 U. S., at 87. We have since held that the duty to disclose such evidence is applicable even though there has been no request by the accused, United States v. Agurs, 427 U. S. 97, 107 (1976), and that the duty encompasses impeachment evidence as well as exculpatory evidence, United States v. Bagley, 473 U. S. 667, 676 (1985). Such evidence is material "if there is a reasonable probability that, had the evidence been disclosed to the defense, the result of the proceeding would have been different." Id., at 682; see also Kyles v. Whitley, 514 U. S. 419, 433-434 (1995). Moreover, the rule encompasses evidence "known only to po-
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