This article provides a comparative analysis of the law and practice of regional international organizations (rio s). Drawing upon the International Law Association (ila) study and individual regional reports, the article offers a cross-regional account of organizations located in Europe, Eurasia, the Middle East, Africa, Latin America and the Caribbean, and the Asia-Pacific. The article focuses on the main conceptual questions that emerged during the study and reflects on some of the main insights gleaned from the cross-cutting comparisons. The article discusses the concept of ‘regional international organization’ and the debates about the appropriate definition to be used in the Study. The article discusses how international law applies to, and within, regional international organizations, examining issues such as the autonomy of the organization’s internal law. The article shows how regional international organizations have influenced the development of international law, by concluding treaties, contributing or catalysing relevant practice to the formation of customary international law, and producing authoritative ‘subsidiary means’ to identify the law. The comparative assessment allows us to offer reflections on the ‘openness’ of regional international organizations and the conditions under which they can shape, and be shaped by, international law. The article concludes with some starting points for further research on the place of regional international organizations
