In pari delicto is a Latin expression used when both parties are equally at fault in relation to unlawful, fraudulent, or seriously wrongful conduct. In practice, it most often arises where a claimant seeks a remedy (such as damages, restitution, or contribution) but the court finds the claimant’s own illegality or wrongdoing is as serious as, or greater than, the defendant’s.The phrase is not generally defined in statute; its content and limits are developed mainly through case law. Courts in England and Wales, Scotland, Northern Ireland and Ireland use the concept when applying illegality defences, equitable principles, and rules on unjust enrichment, contribution and partnership disputes.Key features include: (i) relative blameworthiness – assessing whether the parties are “in equal fault”; (ii) the effect that serious illegality may bar or limit relief; and (iii) policy considerations, including deterrence of illegal transactions and maintaining the integrity of the legal system.Usage is broadly consistent across the UK and Ireland, though the precise formulation and weight given to in pari delicto can differ depending on the local approach to illegality, equity and public policy in each jurisdiction’s appellate case law.