In law, laches is an equitable defense accusing an opposing party of having "sat on his rights"; as a result of this delay, the delaying party is undeserving of equitable relief. It is a form of estoppel for delay. Laches is not entirely analogous to the expiry of the period allowed under a statute of limitations because laches essentially alleges prejudicial delay and unfairness in the context of a particular situation, whereas statutes of limitation tend to define a specific legally prescribed period of time (after the cause of action has accrued) within which a lawsuit for a particular type of cause of action may be commenced or after which the right to recovery is barred. Moreover, while a lawsuit started within a period covered by a limitation period is valid no matter how long it takes to get to trial, laches can be applied even in a situation where a lawsuit has been started and the delays would otherwise be reasonable. It is generally allowed by a court when a defendant could reasonably have believed that the plaintiff was not going to exercise his or her legal rights and acted on that belief to his or her detriment.

A defense lawyer raising the defense of laches against a motion for injunctive relief (a form of equitable relief) might argue that "the plaintiff comes waltzing in at the eleventh hour" when it is now too late to grant the relief sought, at least not without causing great harm that the plaintiff could have avoided. In certain types of cases (for example, cases involving time-sensitive matters, such as elections), a delay of even a few days is likely to be met with a defense of laches, even where the applicable statute of limitations might allow the class of action to be commenced within a much longer time period.

A successful defense of laches will find the court denying the request for equitable relief. However, even if equitable relief is not available, the party may still have an action at law if the statute of limitations has not run out.

A good example of the defense of laches is the case of British Telecom's attempt to rein in fees for the use of hypertext. Professor Richard Stern discusses the notion in the context of BT [1] (pdf).