In This Article Expand or collapse the "in this article" section Estoppel

  • Introduction
  • Private International Law
  • Selected Cases

International Law Estoppel
Emmanuel Voyiakis
  • LAST REVIEWED: 12 April 2019
  • LAST MODIFIED: 23 March 2012
  • DOI: 10.1093/obo/9780199796953-0058


As it is most commonly described, estoppel is a rule of international law that bars a party from going back on its previous representations when those representations have induced reliance or some detriment on the part of others. Although estoppel is now a firmly established rule of international law and is being invoked and applied in an increasingly wide variety of contexts, international lawyers have yet to uncover all its secrets. On the one hand, contemporary practice shows that arguments about estoppel are as likely to appear in the context of a dispute about international trade or human rights as they are to appear in the context of (more traditional) disputes about title over territory or nationality. On the other hand, important disagreements and unresolved questions still remain about the practical requirements of estoppel (and, in particular, the need for detrimental reliance); its relationship to neighboring concepts, such as acquiescence; its proper categorization among the sources of international law; the significance of common law estoppel and its counterparts in other legal systems for the purposes of international law; and the normative basis of estoppel, especially its relationship with broader principles of international law, such as good faith and equity. The open character of these questions suggests that research on estoppel is likely to flourish in the years to come. Antoine Martin’s L’estoppel en droit international public: Précède d’un aperçu de la théorie de l’estoppel en droit anglais (Martin 1979, cited under As Customary International Law) is still the only generally available monograph on estoppel in international law, but this deficit is offset by a wealth of important articles and book chapters. As a general matter, researchers of estoppel in international law need to cast their net wide in three senses. First, the historical roots of estoppel in the common law tradition make it worthwhile (though probably not absolutely necessary) to explore the variety and development of estoppel in that tradition and the possible counterparts of estoppel in other national legal systems. Second, estoppel is almost always discussed in relation to other, neighboring concepts and principles of international law, so it is essential to take into account not only broad treatments of “general principles” in international law, but also works on acquiescence, preclusion, prescription, waiver, unilateral acts and declarations, the protection of reasonable/legitimate expectations, good faith, equity, and so on. Third, many useful discussions and analyses of estoppel are context-specific, so research on estoppel should also take in some aspects of specialist literature on acquisition of territory, international adjudication, protection of foreign investment, nonviolation complaints under the WTO Dispute Settlement Understanding, and so on. The aim of this article is to give the reader a rounded view of the available literature and, hopefully, to stimulate fresh research.

Common Law and Other Domestic Jurisdictions

The concept of estoppel is most closely associated with the common law tradition. Several studies are available on the various guises that estoppel has assumed in the common law of contract and consideration (including Bower 1977, Cooke 2001, Farnsworth 2000, and Wilken and Villiers 2002, all cited under Related Concepts in the Common Law) as well as its most recent incarnations in the context of equity (Spence 1999, cited under Related Concepts in the Common Law) and administrative law (Steele 2005, Sales and Steyn 2004, both cited under English Administrative Law). At the same time, the core idea of estoppel, namely that one who induces some expectations or reliance in another person may be bound not to disappoint those expectations or that reliance, is perfectly familiar to all legal systems and there is a growing body of comparative research that explores the legal concepts each system uses to give expression to that idea, as discussed in Fauvarque-Cosson 2007, Snyder 1998, Nöcker and French 1990, all cited under Similar Concepts in Other Jurisdictions, and Maggs 2006, cited under Related Concepts in the Common Law.

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