A grand jury is a type of jury, comprised of citizens who are presented with evidence from a state or federal prosecutor (District Attorney or United States Attorney) to determine whether there is probable cause to believe a person committed a crime and should be put on trial. If the grand jury determines there is enough evidence, an indictment will be issued against the defendant. This is known as a “true bill,” and the grand jury is said to have “returned a true bill.” If the grand jury does not believe there is sufficient evidence the accused committed a crime and should be put on trial, it is said to return a “no true bill" or "no bill."

The United States Constitution does not require states to use a grand jury to bring criminal charges against a person—but many states do use grand juries, and some states are required by their state constitution or statutes to use a grand jury to secure an indictment for any felony criminal offense. And the U.S. Supreme Court has interpreted the Constitution to require the federal government to use grand juries for all felony crimes.

Grand jury proceedings are not open to the public and accused persons and their attorneys do not have the right to appear before the grand jury, or to be informed of the proceedings. Witnesses subpoenaed by a grand jury do not have the right to have their attorney present during their testimony. The grand jury proceedings are generally transcribed by a court reporter, but the record is sealed.

Federal grand juries generally consist of 16-23 persons who serve or work on the grand jury for a few days each month for approximately one year—after which a new grand jury is selected by the Federal District Court. At least 12 jurors must agree to return a true bill for indictment of the accused person.

Procedures for the use of grand juries in the state court system vary from state to state and are generally governed by the state’s constitution, and statutes that are often located in the penal or criminal code, the code of criminal procedure, or the government code.

In Arizona, grand juries are used to determine whether there is probable cause to believe that an individual has committed a crime and should be indicted. The grand jury is presented with evidence by a prosecutor, and if they find the evidence sufficient, they issue a 'true bill,' leading to an indictment. If they find the evidence insufficient, they return a 'no bill.' While the U.S. Constitution does not mandate states to use grand juries, Arizona law does require the use of grand juries for felony charges. Grand jury proceedings in Arizona are secretive; they are not open to the public, and the accused and their attorneys do not have the right to appear or be informed about the proceedings. Witnesses cannot have their attorneys present during testimony. The proceedings are recorded but the records are sealed. The size of the grand jury and the length of service can vary, but typically, a supermajority is needed to return a true bill. The specific procedures for grand juries in Arizona are governed by state statutes, which can be found in the Arizona Revised Statutes, particularly in the sections pertaining to criminal procedure.