The United States is virtually the only common law jurisdiction in the world that continues to use the grand jury to screen criminal indictments. Generally speaking, a grand jury may issue an indictment for a crime, also known as a “true bill,” only if it finds, based upon the evidence that has been presented to it, that there is probable cause to believe that a crime has been committed by a criminal suspect. Unlike a petit jury, which resolves a particular civil or criminal case, a grand jury (typically having twenty-three or more members) serves as a group for a sustained period of time in all or many of the cases that come up in the jurisdiction, generally under the supervision of a federal U.S. attorney, a county district attorney, or a state attorney-general and hears evidence ex parte (i.e. without suspect or person of interest involvement in the proceedings).
While all states in the U.S. currently have provisions for grand juries, only half of the states actually employ them and twenty-two require their use, to varying extents. The modern trend is to use an adversarial preliminary hearing before a trial court judge, rather than grand jury, in the screening role of determining whether there is evidence establishing probable cause that a defendant committed a serious felony before that defendant is required to go to trial and risk a conviction on those charges.
California, Florida, and some other states, also use so called civil grand juries, investigating grand juries, or the equivalent, to oversee and investigate the conduct of government institutions, in addition to dealing with criminal indictments. For example, in Tennessee, according to the office of the Davidson County District Attorney:
“It is also the duty of the Grand Jury to inquire into the condition and management of prisons and other buildings and institutions of the county, inquire into the condition of the country treasury, into any abuse of office by state or local officers, and to report the results of its actions to the court.”