Latin Term – Meaning – “Opinion that an act is necessary by rule of law” or opinion of law. It is considered to be a belief that the practice is obligatory. This belief in the mandatory nature the conduct or practice may be termed Opinio Juris.  Importance and Relevance – To decide whether a custom has become a source of International Law or not. What is Custom?  ‘Custom’ is one of the many sources of International Law. It means a long established and commonly adopted practice that has acquired the force of law.  Also finds mention in Article 38 (b) of the ICJ Statute that states that International Courts must apply international customs in their decisions, wherever possible and wherever it is a general practice accepted as law either in domestic laws or treaties or international agreements.  Three types – general, regional and local.  General Customs are followed in most of the places and jurisdictions.  Regional Customs are followed in a particular region. North America may have its own Regional Customs distinct from South America or Asia.  Local Customs have limited influence in a geographical area. India Pakistan and Sri Lanka may have a local custom that may not be followed anywhere else.  The existence of a custom can be deduced from the practice and behaviour of states. Such custom becomes part of International Law when it is a State Practice and opinio juris.  State Practice + Opinio Juris = New Customary Rule of International Law   Four Steps  1. Establish existence usage of a practice or conduct. 2. Existence of State Practice 3. Existence of Opinio Juris 4. 1 +2 + 3 = New Custom If all three are present, the practice or conduct becomes a new customary rule of International Law.  1. Step 1 - How to establish existence of usage of a practice or conduct? When a country contends existence of a practice or conduct, it usually gives evidence in form of newspaper reports, statements by government leaders, mentions particular provisions in some law. 2. Step 2 - If such an existence is established, then the next step is to look into State Practice. State Practice simply means how states behave in practice. This practice can be found in their legislation, judicial decisions, administrative acts, official publications, treaties etc. Basically, it covers any act or statements by a state from which its existence as a customary law may be inferred. 3. Step 3 - Once State Practice is established, then the presence of opinio juris is to be seen. To ascertain such presence of Opinio Juris, the behaviour of the state towards that conduct or practice is seen. If the states make that practice or conduct legally obligatory or codify it or make it a legal right, then that conduct, or practice can be said to satisfy the ingredients of ‘Opinio Juris’  4. Step 4 - If existence of usage of a practice or conduct satisfies the dual requirements of State Practice and Opinio Juris, it becomes a valid ‘custom’ in International Law and the Court may decide accordingly.  Issues faced by the Court in deciding presence of Opinio Juris  1. Countries may exert pressure to make a practice a custom but views of countries with greater power does carry greater weight as politics or power cannot be divorced from law. 2. Unsubstantiated and unilateral claim by a state regarding existence of a custom cannot be accepted. It is the international context that plays a vital role in the creation of custom.  3. Unsubstantiated and unilateral claim by a state regarding the existence of a custom are not accepted. It is the international context that plays a vital role in the creation of custom. 4. In new areas of law, Opinio Juris may be quickly or even instantaneously established because of the newness of the situation involved and lack of contrary rules. However, in traditional areas of law, it is more difficult to establish a custom as it has to be done through series of usages. Custom should mirror the perception of the majority of states. Important Case Laws 1.