A jury is a sworn body of people (jurors) convened to hear evidence, make findings of fact, and render an impartial verdict officially submitted to them by a court, or to set a penalty or judgment. Most trial juries are "petit juries", and consist of up to 15 people. A larger jury known as a grand jury has been used to investigate potential crimes and render indictments against suspects, and consists of between 16 and 23 jurors.

The jury system developed in England during the Middle Ages and is a hallmark of the English common law system. Juries are commonly used in countries whose legal systems derive from the British Empire, such as the United Kingdom, the United States, Canada, Australia, and Ireland. They are not used in most other countries, whose legal systems are based upon European civil law or Islamic sharia law, although their use has been spreading. Instead, typically guilt is determined by a single person, usually a professional judge. Civil law systems that do not use juries may use lay judges instead.

The word jury has also been applied to randomly-selected bodies with other purposes, such as policy juries.

The "petit jury" (or "trial jury", sometimes "petty jury") hears the evidence in a trial as presented by both the plaintiff (petitioner) and the defendant (respondent) (also known as the complainant and defendant within the English criminal legal system). After hearing the evidence and often jury instructions from the judge, the group retires for deliberation, to consider a verdict. The majority required for a verdict varies. In some cases it must be unanimous, while in other jurisdictions it may be a majority or supermajority. A jury that is unable to come to a verdict is referred to as a hung jury.

A grand jury, a type of jury now confined mostly to federal courts and some state jurisdictions in the United States and Liberia, determines whether there is enough evidence for a criminal trial to go forward. Grand juries carry out this duty by examining evidence presented to them by a prosecutor and issuing indictments, or by investigating alleged crimes and issuing presentments. Grand juries are usually larger than trial juries: for example, U.S. federal grand juries have between 16 and 23 members. The Fifth Amendment to the U.S. Constitution guarantees Americans the constitutional right to be free from charges for "capital, or otherwise infamous" crimes unless they have been indicted by a grand jury, although this right applies only to federal law, not state law.

In addition to their primary role in screening criminal prosecutions and assisting in the investigation of crimes, grand juries in California, Florida, and some other U.S. states are sometimes utilized to perform an investigative and policy audit function similar to that filled by the Government Accountability Office in the United States federal government and legislative state auditors in many U.S. states.

In Ireland and other countries in the past, the task of a grand jury was to determine whether the prosecutors had presented a true bill (one that described a crime and gave a plausible reason for accusing the named person).

Another kind of jury, known as a coroner's jury can be convened in some common law jurisdiction in connection with an inquest by a coroner. A coroner is a public official (often an elected local government official in the United States), who is charged with determining the circumstances leading to a death in ambiguous or suspicious cases. A coroner's jury is generally a body that a coroner can convene on an optional basis in order to increase public confidence in the coroner's finding where there might otherwise be a controversy. In practice, coroner's juries are most often convened in order to avoid the appearance of impropriety by one governmental official in the criminal justice system toward another if no charges are filed against the person causing the death, when a governmental party such as a law enforcement officer is involved in the death.