Frustration of purpose is a term used in the law of contracts to describe a defense to an action for non-performance based on the occurance of an unforseen event which makes performance impossible or commercially impracticable. A common situation is that the subject matter of the contract - a house or a car for example - is unintentionally destroyed. Law (a loanword from Old Norse lag), in politics and jurisprudence, is a set of rules or norms of conduct which mandate, proscribe or permit specified relationships among people and organizations, provide methods for ensuring the impartial treatment of such people, and provide punishments for those who do not follow... A contract is any promise or set of promises made by one party to another for the breach of which the law provides a remedy. ...

Generally, the non-performance is not excused. If the seller retained the risk of loss from damage or destruction, then the non-performance will likely be excused. However, if it is the buyer who carries the risk of loss, performance will not not excused. A seller will not be excused for nonperformance of an agreement to deliver a commodity. For example, if A agrees to sell B 100 bushels of corn, and A's own crops are destroyed in an accident, A is still contractually obligated to sell B 100 bushels of corn because A can still obtain the corn elsewhere for the sale.

Frustration of purpose also arises as a defense where one party to the contract dies, if that party was uniquely necessary to the performance of the contract. Passage of a subsequent law that makes performance illegal will also excuse nonperformance under this doctrine.