Discovery in criminal prosecutions (cases) generally refers to (1) the process by which the prosecution (government) gathers evidence to prove its case at trial and (2) the prosecution’s duty to disclose such evidence to the defendant—usually through the defendant’s lawyer or counsel.

The state or federal government’s disclosure of material, exculpatory evidence (tending to disprove the defendant’s guilt) or impeachment evidence (tending to impeach or contradict the prosecution’s witnesses) is part of the Constitutional guarantee of a fair trial and is known as Brady material or a Brady disclosure. See Brady v. Maryland, 373 U.S. 83, 87 (1963); Giglio v. United States, 405 U.S. 150, 154 (1972). Because these are Constitutional obligations, such evidence must be disclosed to the defendant regardless of whether the defendant makes a request for exculpatory or impeachment evidence. Kyles v. Whitley, 514 U.S. 419, 432-33 (1995). But the law does not require the government to disclose such evidence to the defendant before entering into a plea agreement with the defendant. U.S. v. Ruiz, 536 U.S. 622, 629 (2002); Weatherford v. Bursey, 429 U.S. 545, 559 (1977).

The Constitution requires the prosecution to disclose exculpatory and impeachment evidence that is material to a finding of guilt—in other words, when there is a reasonable probability that effective use of the evidence will result in an acquittal. United States v. Bagley, 475 U.S. 667, 676 (1985). Because it is sometimes difficult to determine the materiality of evidence before trial, prosecutors generally must take a broad view of materiality and err on the side of disclosing exculpatory and impeachment evidence. And the disclosure of exculpatory and impeachment evidence extends to law enforcement (police) and other government employees—including employees of agencies other than the prosecuting attorney’s office (U.S. Attorney or District Attorney) such as Child Protective Services.

Some states have laws that go beyond the U.S. Supreme Court’s directives in the Brady case and provide more detailed requirements for the timing and substance of such disclosures. These laws may be located in the state’s court opinions, statutes, or rules of criminal procedure, for example.

In Massachusetts, the discovery process in criminal prosecutions involves the prosecution's collection of evidence to prove its case and the obligation to disclose evidence to the defense. This includes both exculpatory evidence, which may prove the defendant's innocence, and impeachment evidence, which may discredit the prosecution's witnesses. These disclosures are mandated by the U.S. Constitution as interpreted in landmark cases such as Brady v. Maryland and its progeny, which require the prosecution to provide Brady material to the defense without a specific request. While the Constitution does not require the disclosure of such evidence before a plea agreement is made, the prosecution must disclose evidence material to guilt or innocence, taking a broad view of materiality to ensure a fair trial. In Massachusetts, as in other states, these requirements may be supplemented by state-specific laws, court opinions, or rules of criminal procedure that provide additional guidance on the timing and substance of disclosures. The duty to disclose extends to all government employees involved in the case, not just the prosecutor's office.