This part of the guide provides you a basic overview of customary international law as well as instructions and quick links to useful resources for finding introductory materials on the topic of Customary International Law.
"Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation." (Westlaw link to this citation provided below.)
"Custom in its legal sense means something more than mere habit or usage; it is a usage felt by those who follow it as obligatory. There must be a feeling that if the usage is not followed some sort of adverse consequence will probably, or at any rate ought to, fall on the transgressor. In technical language there must be a possible ‘sanction’, though the exact nature of this need not be very distinctly envisaged. Evidence that a custom in this sense exists in the international sphere can be found only by examining the practice of states.
That is to say, we must look at what states actually do in their relations with one another, and attempt to understand why they do it, and in particular whether they recognize an obligation to adopt a certain course." (Oxford Scholarly Authorities on International Law link provided below.)
"§ 102 Sources of International Law
1) A rule of international law is one that has been accepted as such by the international community of states
(a) in the form of customary law;
(b) by international agreement; or
(c) by derivation from general principles common to the major legal systems of the world."
Article 38 of the Statute of the International Court of Justice
1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
This quote illustrates the struggle around the status of Customary International Law.
"...it is vitally important to say that unwritten law...is law. This includes Customary International Law, whatever mistaken publicists may teach, even publicists of the most highly qualified kind, and whatever hypocritical and self-serving governments and their sycophantic intellectual camp-followers may say. There is little enough public order in the intergovernmental world without despising the modicum of intelligent self-ordering that, over long centuries, it has been able to find within itself." (Philip Allott, Interpretation in International Law, 386)