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 final of four chapters that consider international legal rules, here those that fall outside the previous chapters’ scopes. Multilateral agreements govern activities and impacts in areas beyond national jurisdiction, including Antarctica, outer space, and the oceans. The comprehensive UN Convention on the Law of the Sea governs solar geoengineering that would take place in the marine environment or would likely result in deleterious effects there. The parties to the Convention on Biological Diversity, a far-reaching agreement, have issued three nonbinding decisions concerning geoengineering, the only such negotiated consensuses from representatives of most countries. The Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques proscribes solar geoengineering's hostile use. Among the four major international legal forums that help resolve disputes in broad ranges of issues, the UN Security Council could address problematic or contentious solar geoengineering.