Grand jury, in law, body of persons selected to inquire into crimes committed within a certain jurisdiction. It usually comprises a greater number than the trial, or petit (also, petty) jury, having since early common law days had between 12 and 23 members. In the United States, federal grand juries have between 16 and 23 jurors. The grand jury receives complaints and accusations in criminal cases, hears evidence adduced by the state, and approves an indictment when satisfied that there is enough evidence against the accused to warrant a trial. It was not until the 17th cent. that the grand jury acquired its modern functions as a check on the discretion of prosecutors and a way of preventing unjustified and politically motivated prosecutions. Grand juries have investigative functions as well, and are sometimes impaneled to issue reports on, e.g., suspected official wrongdoing.

The rules governing grand jury proceedings are very different from those governing trials by (petit) jury. The public is not admitted to hearings, and witnesses can be compelled to testify. The procedure is inquisitorial rather than adversarial: the defense is not allowed to call witnesses, and the prosecutor is not obliged to present both sides of the case. Hearsay and other evidence that might be excluded at a jury trial may be introduced.

The use of grand juries has declined in the 20th cent., in part because they were perceived as prone to either prosecutorial domination or abuse of their investigatory role. Britain abandoned them in the 1930s, and today fewer than half of U.S. states employ them. The information, a written statement issued by a prosecutor, has largely replaced the indictment. The Fifth Amendment to the U.S. Constitution, however, guarantees a grand jury inquiry to anyone accused in federal court of a capital “or otherwise infamous” (i.e., a felony) crime.