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 Montana, grand juries are not commonly used, but they are available under state law. The Montana Code Annotated (MCA) provides for the impaneling of grand juries in certain circumstances. According to MCA Section 46-4-301, a district court judge may order a grand jury to be summoned if it is in the public interest. The grand jury in Montana is composed of 6 to 23 members, and at least 12 jurors must concur to return an indictment, which is known as a 'true bill.' If the grand jury finds insufficient evidence to indict, they return a 'no bill.' Unlike the federal system, where the use of grand juries for all felony offenses is constitutionally required, Montana does not mandate the use of grand juries for all felony charges. Grand jury proceedings in Montana are secret, and the accused and their attorneys do not have the right to appear before the grand jury or to be informed of the proceedings. Witnesses called before the grand jury cannot have their attorneys present during testimony. The records of the proceedings are sealed, similar to federal grand jury proceedings. The use of grand juries in Montana is governed by state statutes, particularly those found in the Montana Code Annotated.