The research guides and books on this page introduce the concepts of customary international law, state practice, opinio juris, and jus cogens. Some of them, in particular, Winer, Kuehl & O'Brien and the Nutshell, also suggest a plan & strategy for doing the research. As with any new legal research project, an excellent starting place can be secondary sources, books and articles that might discuss CIL on your topic. See the sections in this guide for Books, working papers, etc., and Law reviews, journals, articles. Use the other tabs of this research guide to locate any specific sources that you need, e.g., sources for researching the practice of international law, U.S. practice of international law, treaties, case law/jurisprudence, international organizations, etc.