As part of their obligation to pursue justice and promote fairness, prosecutors have a duty to reveal “favorable” information to the defense, as explained below.
For information on discovery law generally, see What and When the Prosecution Must Disclose. For information on trying to make sure the prosecution turns over favorable information, see How can I find out if the prosecution is hiding helpful evidence?
Prosecutors have to provide defendants with favorable information, which includes exculpatory and impeachment evidence.
Information is exculpatory if it is known to the prosecution and tends to prove that a defendant isn’t guilty. (Brady v. Maryland, 373 U.S. 83 (1963).) It must be “material”—that is, likely to have changed a trial’s outcome had the prosecution disclosed it.
Information qualifies as impeachment evidence if it tends to discredit a prosecution witness. For example, prosecutors have to turn over to defendants:
However, the government doesn’t have to disclose impeachment information prior to entering plea agreement with a defendant. (United States v. Ruiz, 536 U.S. 622 (2002).)
To qualify as exculpatory, information doesn’t have to be so powerful that it proves the defendant conclusively innocent. As long as information known to the prosecution might contribute to doubt about the defendant’s guilt in the mind of a reasonable person, the prosecutor must reveal it.
If a prosecutor fails to turn over significant favorable information and the defendant is convicted, an appellate court can overturn the conviction. However, prosecutors are totally immune from personal liability if they don't turn over favorable evidence to the defense. (Van de Kamp v. Goldstein, 555 U.S. 335 (2009).) This means that defendants who have been convicted and imprisoned due to a prosecutor’s failure to disclose information can’t sue the prosecutor in civil court.