Bail is an amount of money that a criminal defendant may be ordered to pay before being released from custody pending trial. Its purpose is to ensure a defendant's return at subsequent trial proceedings. Bail is typically determined during a defendant's first appearance in court. A judge or other court officer sets the amount and conditions of bail. At a bail hearing, a judge has three options:

• Release the defendant on his or her own recognizance or upon an unsecured appearance bond
• Deny bail to the accused
• Set terms of bail, including the amount of bail and any special conditions for release

In common usage, bail typically refers to criminal proceedings. However, in rare instances bail may be imposed in civil cases. Civil bail is used to directly or indirectly secure payment of a debt or to secure a performance of a civil duty. For example, bail may be employed in a civil case to arrest someone to prevent them from fleeing to avoid litigation, or it may be used to prevent an unlawful concealment or disposal of assets. The amount of bail set will be based on the probable amount of damages the plaintiff could collect. Sometimes the deposit may be used to pay the judgment to a plaintiff.

Bail law came to the U.S. through English tradition and laws. Even before the adoption of the U.S. Constitution and Bill of Rights, a judiciary act in 1789 guaranteed a right to bail in all noncapital cases. For a person charged with a capital offense (where death is a possible punishment), bail was discretionary, depending upon the seriousness of the offense. The Eighth Amendment to the U.S. Constitution provides, "excessive bail shall not be required." The U.S. Supreme Court has ruled that the Constitution permits holding a defendant without bail pending a criminal trial. No absolute right to bail exists.

Bail is not meant to act as pre-trial punishment or as a fine. Modern bail laws reflect an intentional emphasis on non-monetary methods to ensure a defendant's appearance at trial. This is meant to avoid discrimination against poor defendants.