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What is Customary International Law?
According to Article 38 of the Statute of the International Court of Justice, the second most significant source of international law is international custom. Specifically the ICJ statute states that the court shall apply international custom as “evidence of a general practice accepted as law”. An international lawyer must prove two basic elements for a particular norm to qualify as customary law:
- State practice must be generally consistent
- Practice must occur out of a sense of legal obligation.
Secondary Sources STATING Custom
Max Planck Encyclopedia of Public International Law
This online edition of the Max Planck Encyclopedia of Public International Law covers central and essential topics in international law.
Max Planck Encyclopedia of Comparative Constitutional Law
The Max Planck Encyclopedia of Comparative Constitutional Law provides analytic coverage of constitutional law topics in a comparative context. The encyclopedia articles address a focused range of topics that seek to provide the best coverage of the essence, character, development, and history of constitutional law from a global perspective.
TOOLS FOR LOCATING TREATISES
Developed cooperatively with scholars and librarians worldwide, Oxford Bibliographies offers exclusive, authoritative research guides.
Oxford Scholarly Authorities on International Law
Oxford Scholarly Authorities on International Law contains full-text online editions of market-leading reference works and treatises published by Oxford University Press. Books such as Oppenheim, our Oxford Handbooks in international law, and the Oxford Commentaries on International Law are made available online for the first time, fully searchable and linked by the Oxford Law Citator.
MAJOR SCHOLARLY WORKS
MAJOR INTERNATIONAL JOURNALS
TOOLS FOR LOCATING ARTICLES
INTERNATIONAL LAW COMMISSION (ILC)
Digests of US Practice in International Law
US State Department publishes a series of digests covering U.S. state practice in international law. Here you'll find written views, practices and pronouncements of the US government.
Digest of US Practice in International Law
Same as above, but freely available from US State Department, covers U.S. state practice in international law. Here you'll find written views, practices and pronouncements of the US government.
American Journal of International Law
Use this source to update the Digest of US Practice. In the table of contents, look at "Contemporary Practice of the US Relating to International Law."
Use these sources when starting your research on customary international law.
Primary Sources STATING Custom
Virtually all states have signed the following conventions.
Vienna Convention on the Law of Treaties
Vienna Convention on Consular Relations
Vienna Convention on the Law of Treaties
General Assembly resolutions, in and of themselves, do not state rules of international law. But, they can result in creating customary international law. Use the following sources to locate resolutions.
Best known resolutions that are in the form of an international agreement are:
Examples of UN General Assembly resolutions that are viewed as customary law are:
To locate judicial opinions that contain a statement that there is a a rule of cutomary international law in effect, consult a well-respected treatise that confirms the statement or consult a database of international opinions.
- General international law treatises listed here.
- International law treatises on specialized topics.
When using the databases above, possible search terms and strategies include the following:
- [rule] AND principle AND custom!
- [rule] /p principle /p custom!
- [rule] /s principle /s custom!
Sources for Locating Evidence of State Practice
Even though customary practices are not usually written down, international lawyers will look to written evidence of state practice documented in a variety of sources. The International Law Commission, the Restatement on Foreign Relations, and many "highly qualified publicists" give us some guidance as to where to look for this written evidence.
- Diplomatic Practice
- Domestic Legislation
- Practice of Intergovernmental Organizations
- International Decisions
- Opinions of legal advisors
- Official military manuals
- Policy statements
What are Generally-recognized Principles of Law?
The "general principles of law recognized by civilized nations" is the third source of law according to the Statute of the International Court of Justice.
General principles are legal concepts so fundamental that they can be found in legal systems throughout the world. In researching general principles of international law, consult background sources, international decisions, and domestic legislation. For a more nuanced discussion, see International Judicial Monitor, What are General Principles of International Law?"