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 Iowa, grand juries are used in accordance with both state statutes and the federal legal system. Under Iowa law, a grand jury is composed of seven members and up to three alternates. The grand jury's role is to determine whether there is probable cause to believe that a person has committed a crime and should be charged. If the grand jury finds sufficient evidence, it issues an indictment, known as a 'true bill.' If not, it returns a 'no bill.' While the U.S. Constitution does not mandate states to use grand juries, the federal government is required to use them for all felony charges, as interpreted by the U.S. Supreme Court. Grand jury proceedings in Iowa are confidential, and neither the accused nor their attorneys have the right to be present or informed about the proceedings. Witnesses cannot have their attorneys with them during testimony. The records of the proceedings are sealed. Federal grand juries, which operate in Iowa under federal jurisdiction, typically have 16-23 members who serve for about a year, with at least 12 jurors needed to indict. State grand jury procedures are governed by the Iowa Code, which outlines the specifics of their composition and operation.