Originally from World Arbitration and Mediation Review

I. Introduction

One of the sources of international law listed in Article 38(1) of the Statute of the International Court of Justice (ICJ) is “international custom, as evidence of a general practice accepted as law.”  Rules of customary international law are binding on all states.  They gradually develop over time based on the uniform and consistent practice of a large number of representative states.  For any customary rule to emerge, states must also have the conviction (or the belief) that such practice is required by law (opinio juris).  This so-called “double requirement” has been consistently adopted by decisions of international courts and arbitral tribunals.

This article is part of the author’s recent work on how customary rules are created and how they can be identified in the field of international investment law In this article, the author intends to focus on one specific element regarding the essential requirement of state practice.  Apart from the actual conduct of states in international relations, other “manifestations” of practice include signing/ratifying international treaties (and subsequent practice in relation to these instruments), activities within international organizations, conduct of the executive, legislative, and judiciary branches of government, and statements by states. Scholars typically enumerate lists of the different elements of relevant state practice.