An American criminal justice procedure whereby, in each court district, a group of 16-23 citizens hold an in camera inquiry on criminal complaints brought by the district attorney and decide if there is cause for criminal prosecution, in which case an indictment is issued.
It is the intent of the system, and the random selection process, to constitute a group representative of citizens of the relevant community generally. They have court officers assigned to them, elect a foreman and have access to a judge from time to time but just to guide them over legal hiccups, not to preside over their deliberations (some of the members of the 1999 Monterey, CA Grand Jury pictured).
If a Grand Jury rejects a proposed indictment, decided not to indict, it is known as a "no bill", "no true bill" or an "ignoramus".
If they accept to endorse a proposed indictment it is known as a true bill and the accused must then face a criminal trial on the charge(s) so returned.
Many American states require a Grand Jury's true bill before any person can be formally charged with a felony.
Notably, a Grand Jury does not decide or even offer an opinion as to the guilt or innocence of the prospective defendant but, instead, decides on a majority vote basis whether or not the district attorney has provided enough evidence to justify a criminal trial.
The Grand Jury is designed to be a buffer or a referee between the ever-powerful government (i.e the district attorney), and individuals who are charged with crimes.
It is a specific requirements set out in the US Fifth Amendment:
"No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury".
Grand juries are a peculiarity of the American criminal justice system.
Larger judicial districts usually enlist a standing Grand Jury for a set period of time, such as a year, where they review applications set before them from time to time by the relevant district attorney, who argues for a true bill. These standing Grand Juries, or those which are ad hoc, can sometimes be tasked to conduct inquiries into other matters of public interest such as public works.
All Grand Jurors must meet certain qualifications (such as having never been convicted of a felony) and be able to read and write.
The district attorney usually runs the show at a Grand Jury with normal rules of evidence put on the shelf. Hearsay is allowed, the hearings are held in secret, the accused has no right to present evidence although that right may be granted, or to cross-examine witnesses, or even to be represented by attorney (although some states do allow it), there is no supervision of a judge, the prosecutor is not obliged to present any evidence favorable to the prospective defendant, and unanimity is not required.
Some have estimated that grand juries issues indictments, true bills, 9 times out of 10.
Some states have opted out of the grand jury system, or present an option to the district attorney, and have an alternative that is similar to the Canadian process where the prosecutor describes the alleged offense in a document called "information" and takes that directly to the court for the commencement of proceedings against the accused.
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