Because of its transboundary effects and because states will be the primary actors, large-scale solar geoengineering and its governance are matters of international law. This is the first of four chapters that consider rules from custom, treaty, and principles as well as international organizations. Although there are no international instruments that are legally binding, in force, and specific to solar geoengineering, international law provides both a substantial extant governance framework and a foundation upon which future norms, rules, procedures, and institutions specific to solar geoengineering could be built. This chapter introduces how international law operates and discusses several general international legal principles that would guide its interpretation and development with respect to solar geoengineering. It describes one source of binding international law – that of states’ customary behavior – and what it might mean for solar geoengineering, emphasizing procedural obligations. The chapter reviews some relevant nonbinding multilateral environmental agreements and activities of intergovernmental organizations, such as the Intergovernmental Panel on Climate Change.