A grand jury is a body of citizens, usually in groups of 16 to 23, who decide whether a prosecutor has presented enough evidence to obtain an indictment of an individual. Grand juries are designed to serve as a type of buffer between the prosecution and the defendant. Critics charge that grand juries—more often than not—do not serve this ideal buffering function and instead serve as a rubber stamp for the prosecution. There is a famous saying about grand juries that reflects this sentiment—“a grand jury will indict a ham sandwich.”

Grand juries are distinct from trial juries—or petit or trial juries—which usually consist of 12 people. Sometimes prosecutors use the grand jury method of initiating criminal charges against individuals, rather than filing what an accusatory document—called an information—and proceeding with a preliminary hearing. The grand jury serves as a screening mechanism to determine whether the prosecutor has enough evidence to obtain what is known as a true bill. If the grand jury decides there is no evidence, it would issue what is called a no bill. Grand juries are used in federal court and in some states. Most states explain the operational workings and functions of the grand jury in their rules of criminal procedure.