Grand juries usually have two primary functions: investigation and indictment.

Grand-jury investigations usually involve crimes. However, some states allow grand juries to investigate alleged misconduct of public officials, corruption, prisons, public records, and other public offices or facilities. Investigatory grand juries may subpoena witnesses, invoke the court’s contempt power, and perform other similar duties to obtain information and make a decision.

The indicting function involves screening possible criminal cases for trial or, at a later stage in the proceedings, reviewing the prosecutor's charges and returning a "true bill."  While grand juries have a long legal history, today the term "indictment" usually refers to any grand-jury accusation, whether presented on its own or based on information from the prosecutor.

All states use grand juries in some capacity. Some state grand juries merely exist de jure; Wisconsin, for example, has a grand-jury statute but has not convened a grand jury in decades. 

Most states (all but Connecticut and Pennsylvania, and the District of Columbia) retain the option of a grand-jury indictment. Eighteen states (Alabama, Alaska, Delaware, Georgia, Kentucky, Maine, Massachusetts, Mississippi, New Hampshire, New Jersey, New York, North Carolina, Ohio, South Carolina, Tennessee, Texas, Virginia, and West Virginia) require an indictment to begin felony prosecutions; four (Florida, Louisiana, Minnesota, and Rhode Island) require an indictment to begin prosecutions that could result in life imprisonment or a death sentence. The Fifth Amendment of the U.S. Constitution requires that felony defendants in federal courts be indicted by grand jury. 

Arizona, Florida, Louisiana, Minnesota, Missouri, Montana, and North Carolina limit the scope of the investigatory grand jury to criminal activity that is brought to their attention by the prosecutor or the court. 

Grand juries in Alabama, Alaska, Arkansas, California, Idaho, Indiana, Iowa, Kentucky, Nebraska, Nevada, North Dakota, Ohio, Oklahoma, Oregon, South Dakota, Tennessee, Texas, Virginia, and West Virginia can investigate alleged crimes occurring within their venues (usually the county where the grand jury sits).

Grand juries are being used less often by states. Critics of the system contend that although grand juries were created as a constitutional safeguard against unwarranted charges, grand juries often have unchecked power. The secrecy of proceedings is particularly troublesome to critics. 

Thus, proposed reforms include safeguarding the rights of witnesses, opening up the process, applying the rules of evidence to grand-jury proceedings, and changing procedural rules in accordance with these reforms (or to be more in accordance with the grand jury’s historical roots).

The most comprehensive source on grand juries is Sara Sun Beale et al., Grand Jury Law and Practice, 2nd ed. (St. Paul: West, 1997). The two loose-leaf volumes include a history of the grand jury, as well as information about selecting and summoning grand jurors, secrecy of proceedings, subpoenas, offenses, and grand-jury reports.

Susan Brenner and Lori Shaw, professors at the University of Dayton, have created a comprehensive Grand Jury Web site that includes information about state and federal grand juries.

Table 38, “Grand Juries: Composition and Function,” in State Court Organization, 2004, provides state-by-state information about the grand-jury system, including whether an indictment is required for all felonies; size; size of quorum; number needed to indict; statutory term; and special civil duties.