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.


Sources of international law



UN Declarations and Resolutions

The resolutions and decisions of the United Nations General Assembly and Security Council may provide valuable information. Although they cannot themselves constitute international law or serve as conclusive evidence of a rule of customary law, such resolutions do have value in providing evidence of existing or emerging law. This point of view has been acknowledged by the International Law Commission.

As every Member State has been recognized and has a vote in the UN General Assembly, finding General Assembly resolutions that passed unanimously or near-unanimously may serve as an excellent starting point in the legal research process. Pay attention to the language of the resolution. Those with firm obligations versus those that are merely aspirational or advisory in nature are more likely to be considered valuable as evidence of customary international law.

 Official Document System Search. ODS is the UN's Official Document System. You can search for UN declarations, resolutions and other documents by keywords, then narrow your search.

 Start Your Research: (7) Resolutions and Decisions of International Organizations.

Soft law

Reference works

Selected books and articles

Periodicals, serial publications