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International Law Research: Custom

Starting points to either collect international law sources or conduct in-depth public international law research.

What is International Custom?

In general terms, custom has two components:

  • state practice
  • opinio juris (acceptance as law)

The ILC adopted new Draft conclusions on identification of customary international law in 2018. In summary, according to the draft conclusions (colors show overlap):

“Forms of State practice include, but are not limited to: diplomatic acts and correspondence; conduct in connection with resolutions adopted by an international organization or at an intergovernmental conference; conduct in connection with treaties; executive conduct, including operational conduct “on the ground”; legislative and administrative acts; and decisions of national courts.” (p. 133) (emphasis added)

“Forms of evidence of acceptance as law (opinio juris) include, but are not limited to: public statements made on behalf of States; official publications; government legal opinions; diplomatic correspondence; decisions of national courts; treaty provisions; and conduct in connection with resolutions adopted by an international organization or at an intergovernmental conference.” (p. 140) (emphasis added)

Selected Books Discussing International Custom

Sources Demonstrating International Custom

Evidence of custom includes, in part, information from secondary sources referencing custom, treaty provisions, cases by national courts, other laws of individual states, and publications by a country's state department, foreign affairs ministry, external affairs ministry, or a similar government department focused on international relations. Additional sources to consult for evidence of custom include:

U.S. Custom

Selected sources for evidence of custom in the U.S. include the following. Discover many additional sources through Sources of State Practice in International Law (above) and Researching Customary International Law, State Practice and the Pronouncements of States Regarding International Law (below).

Sources of Custom for Selected Topics

International Humanitarian Law
International Criminal Law
International Investment Law

Additional Guides