According to Article 38 of the International Court of Justice Statute, customary international law and general principles of law are two sources of international law. These are also the two most difficult sources to research because they are documented in such a wide variety of sources.
Article 38 also mentions judicial decisions and the teachings of publicists as "subsidiary means for the determination of rules of law." These are not authorities, rather they are authoritative evidence of the state of international law.
Guide to International Legal Research (Newark, NJ: LexisNexis Matthew Bender, 2002-).
S. Rosenne, Practice and Methods of International Law (New York: Oceana, 1984). A very thorough treatment of the topic and the resources.
C. Germain, Germain's Transnational Law Research (Ardsely-on-Hudson, NY: Transnational Juris Publications, 1991-).
Contemporary Practice of Public International Law (Schaffer and Snyder, eds., New York: Oceana, 1997).
Sources of State Practice in International Law (Gaebler & Smolka-Day eds., Ardsley, NY: Transnational Publishers, 2001-).
Researching Public International Law (Columbia Law Library). Unlike most such research guides, this one does provide some guidance on researching custom and state practice as well as general principles.
Research Guide to Customs, General Principles and the Teachings of Highly Qualified Publicists (Oklahoma City University Law Library).
Customary International Law
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What is customary international law?
"It consists of rules of law derived from the consistent conduct of States acting out of the belief that the law required them to act that way." (Rosenne, Practice and Methods of International Law, p. 55)
The elements of customary international law include:
1. Widespread repetition by States of similar international acts over time (State practice). 2. Acts must occur out of sense of obligation (opinio juris). 3. Acts must be taken by a significant number of States and not be rejected by a significant number of States.
"Customary international law develops from the practice of States. To international lawyers, ?the practice of states' means official governmental conduct reflected in a variety of acts, including official statements at international conferences and in diplomatic exchanges, formal instructions to diplomatic agents, national court decisions, legislative measures or other actions taken by governments to deal with matters of international concern." (Public International Law in a Nutshell, pp. 22-23).
A peremptory norm (also called "jus cogens") is an international law rule or principle which is accepted and considered binding by the international community. Generally, these norms include prohibitions on use of force, crimes against humanity, war crimes, piracy, genocide, and slavery. For a thorough discussion of this issue, see Peremptory Norms in International Law(Oxford; New York, N.Y.: Oxford University Press, 2006). "Custom is the most commonly recognized source of peremptory norms." (See pp. 113 of Peremptory Norms in International Law).
For a good analysis of the customary international law, see Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, 95 Am. J. Int'l. L. 757 (2001). Contains references to other standard sources of customary international law.