The purpose of criminal proceedings is to determine the truth of what happened. The ultimate goal is that no innocent person should be convicted a crime he did not commit. Toward that end, the California Constitution and the California Penal Code both contain provisions for discovery of evidence in criminal cases.

Once a defendant is arraigned, his attorney can start the "informal" discovery request process. The prosecution will then be required to hand over copies of its evidence including police reports, statements from witnesses, audio and video recordings of pertinent events, etc. The defense has a limited duty to hand over evidence to the prosecutors, too, but the evidence required to be disclosed is usually limited to items that will be admitted during trial (e.g., physical evidence, contact info for defense witnesses and their statements, etc.).

Although failure of the defense or the prosecution to disclose required evidence can lead to sanctions for the attorneys and possibly to a mistrial or dismissal being declared for the case, these are rare outcomes. More typically, the court will reprimand an attorney failing to disclose evidence and give more time to the other party to prepare the case based on that evidence.