Estoppel

Estoppel is a judicial device in common law legal systems whereby a court may prevent or "estop" a person from making assertions or from going back on his or her word; the person being sanctioned is "estopped".Estoppel may prevent someone from bringing a particular claim. Legal doctrines of estoppel are based in both common law and equity. It is also a concept in international law.

Estoppel is sometimes said to be a rule of evidence, whereby a person is barred from leading evidence of a fact that has already been settled or they are otherwise precluded from asserting, but that may be an oversimplification. Firstly, although some estoppels relate to preventing a party from asserting facts, others relate to preventing a party from asserting a right or a claim. Secondly, under the conflict of laws in common law jurisdictions, matters of evidence are usually treated as procedural matters for the law of the local court (the lex fori), whereas it is generally accepted that an estoppel may affect substantive rights and are therefore matters to be determined by the proper law (or lex causae) that governs the particular issue.

There are many different types of estoppel that can arise under common law legal systems. It has been judicially noted on more than one occasion that the link between them is often somewhat tenuous. Treitel on Contracts notes that "unconscionability...provides the link between them", but they nevertheless have "separate requirements and different terrains of application". The courts have long abandoned an attempt to create a single general underlying rationale or principle:

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