The most important case for criminal discovery is Brady v. Maryland (1963). Brady established an accused’s due process right to ‘favorable evidence that is material to guilt or punishment.’ Material evidence is defined as evidence that would create a reasonable doubt that otherwise did not exist. It is significant that Brady requested evidence that his co-defendant confessed to committing the murder.
In 1976 Linda Agurs was convicted of stabbing her lover in a cheap hotel room. She made no request for discovery. After she was convicted of murder she discovered the victim had two convictions for carrying illegal knives and for assault. The United States Supreme Court held the prosecution had a duty to disclose this evidence to Agurs, US v. Agurs, 427 U.S.97. To win a Brady challenge an accused must show the hidden evidence would have been helpful to the defendant.