In the 1963 decision in Brady v. Maryland, the U.S. Supreme Court held that in a criminal case, the accused has a constitutional right to discover exculpatory evidence held by the prosecution. The doctrinal seeds of the Brady decision rest on the due process clause and almost 30 years of prior case law addressing such issues as witness intimidation, perjured testimony, impeachment evidence, and informant identity. The major standard applied in discovery cases -- materiality -- was defined in the 1985 decision in United States v. Bagley. The discovery right is limited to evidence in the Government's possession that is both favorable to the defense and material to the issue of guilt or innocence. The Brady rule imposes a substantial burden on police and prosecutors to be alert to the existence of such evidence and to be sensitive to their duty to disclose. A request for disclosure is not necessary to trigger this obligation in all cases, and the duty operates regardless of the good faith of the prosecution. A violation of the duty, if discovered, results in the invalidation of the proceeding and requires a new trial. An undetected violation results in an unfair trial and a violation of due process. 57 footnotes.