Compurgation, also called trial by oath, wager of law, and oath-helping, was a defence used primarily in medieval law. A defendant could establish their 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.