The circumstances under which such nondisclosures will be sufficient grounds to vitiate a conviction of the defendant on appeal or in collateral proceedings will be examined. This article outlines the development of the Brady rule, the underlying purpose of which is the ascertainment of the truth and assurance that the accused receives a fair trial. Two significant Supreme Court rulings defining and redefining the standard of materiality to be applied by appellate courts to defense claims of prosecutorial nondisclosure are examined. The article also focuses on the prosecutor's ethical and constitutional obligations to disclose evidence favorable to the defendant. The author discusses whether Supreme Court decisions setting forth a new standard of materiality for Brady violations have actually limited defendants' rights and advocates the adoption of a different standard that would stress the nature of the withheld evidence rather than just its effect on the proceeding. 317 notes