Customary International Law

Abstract

According to Article 38 of its Statute, the International Court of Justice 'whose function is to decide in accordance with international law such disputes as are submitted to it,' has to apply, inter alia, 'international custom.' This source of public international law is described, in the same Article, as 'evidence of a general practice accepted as law.'

Text

According to Article 38 of its Statute, the International Court of Justice 'whose function is to decide in accordance with international law such disputes as are submitted to it,' has to apply, inter alia, 'international custom.' This source of public international law is described, in the same Article, as 'evidence of a general practice accepted as law.' This description of international custom, even though it has been criticized for its exact formulation, at least makes clear that international custom generally refers to a description of State practice. But only when such practice is accepted by the States themselves as legally required, it is considered customary law. Once a certain practice is understood to be customary law, States are obliged to act as the rule of customary international law prescribes.

International customary law is probably the most disputed and discussed source of international law. For example, it is not clear when a particular State practice becomes a legally binding State practice. It is also unclear how one can identify a rule of international custom, or how one can prove its existence. Important elements are the manifestations of State practice, like declarations, statements or other official documents.

The International Law Commission appointed Sir Micheal Wood as Special Rapporteur concerning the issue of Customary International Law. His reports and recommendations are available in the bibliography under documents.

This Research Guide is intended as a starting point for research in the field of Customary International Law. It provides the basic materials available in the Peace Palace Library, both in print and electronic format. Handbooks, leading articles, bibliographies, periodicals, serial publications and documents of interest are presented in the Selective Bibliography section. Links to the PPL Catalogue are inserted. The Library's subject heading (keyword) International Customary Law is instrumental for searching through the Catalogue. Special attention is given to our subscriptions on databases, e-journals, e-books and other electronic resources. Finally, this Research Guide features links to relevant websites and other online resources of particular interest.

Librarian’s choice
Publication
D'Aspremont, J., The Discourse on Customary International Law, Oxford, Oxford University Press, 2021.
Along with treaties, custom is one of the sources of international law. It is known to consist of two elements: state practice and opinio juris. While many studies have looked at traditional questions of how to identify customary law, this book takes a new and original approach.
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Sources of international law

Treaties

Case-law

UN Declarations and Resolutions

Soft law

Reference works

Selected books and articles

Periodicals, serial publications

Bibliographies