This introductory paper to the symposium hosted by the Leiden Journal of International Law, and edited by the authors, dealswith the function of domestic courts as agents for the development of international law. The paper 'sets the scene' for the contributions to the symposium, which seek to trace the impact of domestic courts in the development of canonical areas of international law, such as jurisdiction, immunity, state responsibility, the law of international organizations/human rights, and the law of armed conflict/conduct of hostilities. It discusses the formal quality and actual influence of domestic-court decisions on the development of international-law, and introduces the concept of 'agents' of international-law development. This is the analytical perspective that the contributions to the symposium adopt.