Compurgation, also called trial by oath, wager of law, and oath-helping, was a defence used primarily in medieval law. A defendant could establish his innocence or nonliability by taking an oath and by getting a required number of persons, typically twelve, to swear they believed the defendant's oath. The wager of law was essentially a character reference, initially by kin and later by neighbours (from the same region as the defendant), often 11 or 12 men, and it was a way to give credibility to the oath of a defendant at a time when a person's oath had more credibility than a written record. It can be compared to a legal wager, which is the provision of surety at the beginning of legal action to minimize frivolous litigation.

Compurgation was found in early Germanic law, in early French law (très ancienne coutume de Bretagne), in Welsh law, and in the English ecclesiastical courts until the seventeenth century. In common law it was substantially abolished as a defence in felonies by the Constitutions of Clarendon in 1164. The defence was still permitted in civil actions for debt and vestiges of it survived until its statutory repeal at various times in common law countries: in England in 1833, and Queensland at some point before the Queensland Common Practice Act of 1867 which makes direct reference to the abolition of wager of law.

The word compurgation is composed of Latin, com "with" and purgare "to make clean, cleanse, excuse". Latin com- is also an intensifier and turns a word into the superlative form, so compurgation, by etymology, means "to thoroughly clean or excuse", and is cognate with purge.

The procedure in a wager of law is traced by Blackstone to the Mosaic law, Exodus 22:10–15; but it seems historically to have been derived from the system of compurgation, introduced into England from Normandy, a system which is now thought to have had an appreciable effect on the development of the English jury. It also has some points of resemblance, perhaps some historical connection, with the sponsio and the decisory oath of Roman law, and the reference to oath of Scottish law.

[Compurgation] had originated in Anglo-Saxon England in the ties of kinship that bound people together in the period before the year 1000, a time when each man was responsible for the acts of his blood relatives. Later, kinship gave way to a more tribal affiliation and a loyalty to the place of one's birth. When disputes more often than not led to violence, it seemed natural that neighbors would band together. They aligned themselves with a neighbour who was accused in court and swore that in good conscience they believed he was telling the truth. The number of oath-helpers required depended on the defendant's rank and the character of the lawsuit. Eventually it became standard practice to bring eleven neighbours into court to swear for the defendant. The oath-helpers were called compurgators, and the wager of law was called compurgation.

After the defendant have taken an oath of innocence, their guilt or innocence is determined by a means named "trial by ordeal".

Supervised by a priest, the defendant is asked to hold a red-hot iron bar in his or her hand, or to take a stone out of a boiling boiler. If the defendant's hand recovers normally, he or she is innocent; if it does not, he or she is guilty.

The wager of law, also called compurgation, is an old legal practice, dating back to Saxon and feudal times, which was contemporaneous to the appeal to God to prove fact by trial by battle (wager of battle, trial by combat, or judicial duel), and of trial by ordeal.