An illegal agreement, under the common law of contract, is one that the courts will not enforce because the purpose of the agreement is to achieve an illegal end. The illegal end must result from performance of the contract itself, however. A contract that requires only legal performance, such as the sale of packs of cards to a known gambler, where gambling is illegal, will nonetheless be enforceable. A contract to pay a gambling debt, however, will not.

A famous example in the United States is Bovard v. American Horse Enterprises 247 Cal. Rptr. 340 (1988), in which the California Supreme Court refused to enforce a contract for payment of promisory notes used for the purchase of a company that manufactured drug paraphenalia.