The main purpose of a grand jury is to hear evidence presented by the prosecutor and decide whether or not there is probable cause for an indictment. Unlike a regular jury trial, the grand jury will only hear conflicting or contrary evidence if a prosecutor wishes to present it in good faith. For this reason, modern grand juries are viewed as a prosecutor's best friend, but this was not always the case. In the early twentieth century, there were instances of runaway juries in courthouses across America.

A runaway jury occurs when grand jurors ignore the prosecutor and take it upon themselves to investigate the crime. One such jury in New York in the 1930's. It actually blocked prosecutors from entering the grand jury room and began its own investigation into political corruption. Today, prosecutors are in firm control of the proceedings and there is rarely a threat of having a runaway jury.

The Fifth Amendment requires that an indictment on federal criminal felony charges be brought by a grand jury. This requirement is not binding on the states, so a state may or may not use a grand jury. A federal grand jury must have between 16 and 23 people. There must be a minimum of 16 jurors in the grand jury room in order for the jury to hear evidence.

The minimum number necessary for a grand jury to convene is called a quorum. The grand jury is composed or made out of the same pool of people as the trial jury. They are selected from voter registration lists, motor vehicle records, and other public records. Unlike trial jurors, grand jurors are not questioned for impartiality, or lack thereof.

There are two types of grand juries, regular and special. Regular grand juries sit for a maximum of 24 months, while special grand juries can sit up to three years. Regular grand juries can bring indictments in all types of cases, but special grand juries only hear evidence regarding organized crime. Organized crime could be defined as drug activity, government corruption, or the more typical Mafia crime.

Although both trial juries and grand juries are sworn in by a judge, the similarity in proceedings ends there. There is no judge present in the grand jury room during proceedings. Defense attorneys are not allowed in the grand jury room. The grand jurors hear the case from the prosecutor's point of view.

The prosecutor decides which witnesses to call and if anyone will receive immunity in exchange for their testimony. The prosecutor can subpoena anyone to appear before the grand jury without probable cause. Once subpoenaed by the prosecutor, a witness must assert a privilege to avoid having to appear. For example, marital privilege between spouses, or lawyer-client privilege would exempt a witness from testifying.

Rule 6(e) of the Federal Rules of Criminal Procedure mandates that all grand jury proceedings be secret. The prosecutor, grand jurors, and court stenographer must keep the court proceedings a secret. Keeping the grand jury proceedings secret protects the jurors from outside influences, witness tampering, and prevents a suspect from fleeing upon learning he or she is about to be indicted. The grand jury is a powerful weapon in the hands of a skilled prosecutor.