This article considers the complex and controversial issue of climate geoengineering, examining the international legal framework for regulating large-scale interventions in the Earth’s natural climate system to offset emissions and to avoid catastrophic climate change. It uses the injection of sulphate aerosols into the stratosphere and ocean iron fertilization as examples. It sets out the fragmented nature of the international legal framework which might regulate geoengineering, and the contours of any possible future legal response. The article concludes that the emergence at the international level of a single treaty dedicated to the regulation of all geoengineering methods is both unlikely and undesirable, favouring instead an approach based on a number of guiding principles for the governance of geoengineering research which are briefly elaborated. It suggests these could be applied against the backdrop of a general prohibition on deployment pending the fuller development of appropriate governance frameworks for specific geoengineering methods.