Before the government can charge you with a serious federal crime, it usually has to convince a group of ordinary citizens, in secret, that it has enough to proceed. That body is older than the United States.
A grand jury is a group of citizens convened to decide whether there is enough evidence to formally charge someone with a serious crime. It does not decide guilt; it decides only whether the government may proceed to trial, by issuing an indictment.
It is required by the Fifth Amendment for serious federal crimes, which states that no person shall be held to answer for a capital or otherwise infamous crime except on indictment of a grand jury. It is a screen between the citizen and the prosecutor.
Its roots are deeply English, traced to the Assize of Clarendon in 1166 under King Henry II. The grand jury was meant to be a check, a buffer of ordinary people standing between an individual and the raw power of the crown to accuse.
It is larger than a trial jury, often around twenty-three people, hence grand, French for large, versus the smaller petit or trial jury. It works in secret, hears only the prosecutor's side, and votes on whether to indict.
