In law, desuetude is a doctrine that causes statutes, similar legislation, or legal principles to lapse and become unenforceable by a long habit of non-enforcement or lapse of time; it is what happens to unrepealed laws when they become obsolete.

The doctrine of desuetude is not favoured in the common law tradition. In 1818, the English court of King's Bench held in the case of Ashford v. Thornton that trial by combat remained available at a defendant's option in a case where it was available under the common law. The concept of desuetude has more currency in the civil law tradition, which is more regulated by legislative codes, and less bound by precedent.