The function of a grand jury is to accuse persons who may be guilty of an offense, but the institution is also a shield against unfounded and oppressive prosecution. It is a means for lay citizens, representative of the community, to participate in the administration of justice. It can also make presentments on crime and maladministration in its area. The traditional number of the grand jury is 23.
The mode of accusation is by a written statement in solemn form (indictment) describing the offense with proper accompaniments of time and circumstances, and certainty of act and person or by a mode less formal, which is usually the spontaneous act of the grand jury, called presentment. No indictment or presentment can be made except by concurrence of at least twelve of the jurors. The grand jury may accuse upon their own knowledge, but it is generally done upon the testimony of witnesses under oath and other evidence heard before them. The proceedings of grand jury are, in the first instance, at the instigation of the government or other prosecutor, and ex parte and in secret deliberation. The accused has no knowledge nor right to interfere with their proceedings.
The Fifth Amendment to the Constitution of the United States reads, “No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a grand jury.
While all states currently have provisions for grand juries today approximately half of the states employ them and 22 require their use, to varying extents.The constitution of Pennsylvania required, between 1874 and 1968, that a grand jury indict all felonies.