A grand jury is a jury—a group of citizens—empowered by law to conduct legal proceedings, investigate potential criminal conduct, and determine whether criminal charges should be brought. A grand jury may subpoena physical evidence or a person to testify. A grand jury is separate from the courts, which do not preside over its functioning.

Originating in England during the Middle Ages, grand juries are only retained in two countries, the United States and Liberia. Other common law jurisdictions formerly employed them, and most others now employ a different procedure, referred to as a preliminary hearing, that does not involve a jury. Grand juries perform both accusatory and investigatory functions. The investigatory functions of grand juries include obtaining and reviewing documents and other evidence, and hearing sworn testimonies of witnesses who appear before it; the accusatory function determines whether there is probable cause to believe that one or more persons committed a particular offense within the venue of a district court.

In Ireland, for a period, they also functioned as local government authorities:

"They fixed the salaries of public officers; they regulated prisons and houses of correction; they levied funds for the support of hospitals; they made and repaired roads and bridges, and they framed accounts of the expenses incurred in these matters... They determined what public works should be undertaken — what price should be paid for them, and who were the individuals that should undertake them, and be responsible for their completion. They settled the amount of the local taxation of the county, and, under their direction, it was levied from the actual occupiers of the land."

In Japan, the Law of July 12, 1948, created the Kensatsu Shinsakai (Prosecutorial Review Commission or PRC system), inspired by the American system.

The grand jury (from the French word grand meaning "large") is so named because traditionally it has more jurors than a trial jury, sometimes called a petit jury (from the French word petit meaning "small"). A grand jury in the United States is usually composed of 16 to 23 citizens, though in Virginia it has fewer members for regular or special grand juries.