Customary International Law
“Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation.”
This definition was published in §102 (2) of the Restatement of the Law, Third, Foreign Relations Law of the United States, published by the American Law Institute in 1987. The Restatement’s reporters’ notes for this section state that “No definition of customary law has received universal agreement, but the essence of Subsection (2) has wide acceptance” and goes on to explain various difficulties in defining custom.”
When is state practice considered to be customary international law? The Restatement requires both:
J.L. Brierly describes it as follows: “Custom in its legal sense means something more than mere habit or usage; it is a usage felt by those who follow it to be an obligatory one. There must be present a feeling that, if the usage is departed from, some form of sanction probably, or at any rate ought to, fall on the transgressor.”Obviously, terms such as “a feeling that”, “will probably” and “ought” are difficult to prove. As Mark Janis puts it in his book, An Introduction to International Law, “The determination of customary international law is more an art than a scientific method.” From Globalex, Researching Customary International Law, State Practice and the Pronouncements of States regarding International Law by Silke Sahl
(1) Whether a rule has become international law is determined by evidence appropriate to the particular source from which that rule is alleged to derive (§ 102).
(2) In determining whether a rule has become international law, substantial weight is accorded to
(a) judgments and opinions of international judicial and arbitral tribunals;
(b) judgments and opinions of national judicial tribunals;
(c) the writings of scholars;
(d) pronouncements by states that undertake to state a rule of international law, when such pronouncements are not seriously challenged by other states.
Jus Cogens: Black's Law Dictionary Deluxe Ninth Edition
Video Link (40 minutes)
Source: United Nations Audio Visual Library, Customary International Law: Jus Cogens
Judge A. A. Cançado Trindade, International Court of Justice
Codification Division, Office of Legal Affairs
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Customary Law Can Be Difficult to Research: Strategies for Success
Restatement of the Law, Third by 
Sources of State Practice in International Law by Besides searching digests and yearbooks, Foreign Relations of the United States is a primary way of searching for practices of the United States. Also consider the Restatement of the Law, Third, Foreign Relations.
Yearbooks and digests may have demonstrate international and state practice and thus constitute evidence of international law.
Digests and yearbooks are a primary way of discerning the practice of states.
Treatises can constitute the evidence of publicists on international law. Which treatises are such evidence is open to interpretation.
Custom, Power and the Power of Rules by 
International Law by 
Oppenheim's International Law by 
Principles of Public International Law by subscription and free databases, ejournals and ebooks
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