Solar geoengineering is presently being researched through indoor work and outdoor experiments without potential transboundary impacts. Existing national and subnational legislation and common law govern such activities, including for their environmental risks, and is an essential component of the existing governance framework within which solar geoengineering is developing. To offer a case study, this chapter reviews applicable American law. The United States was chosen because its environmental legal regime is among the most elaborate and influential, and because solar geoengineering research is presently moving forward most rapidly there. The text considers three major pieces of federal environmental legislation: the Clean Air Act, the National Environmental Policy Act, and the Endangered Species Act. Liability for harm, which is found in the common law of torts, is briefly introduced. Thereafter, the chapter describes relevant laws regarding weather modification and marine pollution, as well as a federal geoengineering bill. A salient question throughout this concerns the extraterritorial application of US law.