Promissory Estoppel

Promissory estoppel is an equitable doctrine used primarily in common law jurisdictions to prevent injustice when one party has relied on another's promise, even though the promise lacks the formal elements of a binding contract (such as consideration). Courts apply it to "estop" — that is, bar — the promisor from denying the promise when going back on it would harm the party who reasonably relied on it.

The classic formulation in English law comes from Central London Property Trust Ltd v High Trees House Ltd [1947] KB 130, in which Lord Denning held that a landlord's promise to reduce wartime rent was binding on the landlord for the period of reliance, even without fresh consideration. In the United States, the doctrine is codified in the Restatement (Second) of Contracts § 90, which provides that a promise is enforceable if the promisor should reasonably expect it to induce action or forbearance, it does induce such reliance, and injustice can be avoided only by enforcement.

Typical elements courts look for are:

• A clear and unambiguous promise
• Reasonable and foreseeable reliance by the promisee
• Actual reliance that causes detriment
• Injustice that can only be cured by enforcing the promise

In English law, promissory estoppel is generally treated as a "shield, not a sword" — it can defend against enforcement of strict legal rights but does not by itself create a new cause of action. In American law, by contrast, it is often used affirmatively to recover reliance damages where no contract exists.

The doctrine has analogues in public international law, where concepts like estoppel and acquiescence operate when states rely on another state's representations — as discussed by the International Court of Justice in the Temple of Preah Vihear case (Cambodia v. Thailand, 1962) and the North Sea Continental Shelf cases (1969). Though distinct from the contract doctrine, the underlying principle of protecting reasonable reliance is similar.