In the field of international law, international custom and general principles of law refer to recognized legal norms that have developed through the interactions between states over a period of time. The concept implies that states are expected to carry out their affairs consistently with past-accepted conduct. Many international treaties are attempts to codify existing customary law and general principles of law.
Customary international law and general principles of law are the two most difficult sources to research because they are unwritten rules. Instead, they are documented in such a wide variety of materials. As such, secondary sources are useful places to start your research.
To find customary law and general principles of law, start by searching for relevant texts and treatises, law review articles, and judicial decisions from national and international tribunals. Statements made in landmark resolutions and declarations of the UN can also be helpful. Legal encyclopaedias may also contain relevant materials.
Researching Customary International Law is an open access tutorial from the Institute of Advanced Legal Studies by Hester Swift. It provides an overview of the key print and online sources for researching customary international law. It covers finding evidence of state practice in the records of states' foreign relations and diplomatic practice, and in legislation concerning international obligations. It also looks at researching the practice of the UN Security Council, UN General Assembly and UN Human Rights Committees.
An online database that contains full-text online editions of classic publications, leading reference works, and authoritative books published by Oxford University Press, such as Oppenheim's International Law and the Oxford Commentaries on International Law series. The books are fully searchable and linked by the Oxford Law Citator. The books can also be browsed by author or subject.
The Collected Courses of the Hague Academy of International Law (also known as Recueil des Cours)
This online encyclopaedic collection of international law topics is published by BRILL, and includes more than 350 volumes, containing over 1200 lectures given by prominent legal specialists since 1923. At least seven new volumes are added every year. All courses are published in the language in which they were delivered, either in English or in French. The database is searchable and can also be browsed.