The precautionary principle is used in arguments for, as well as against, climate engineering: On the one hand, the principle can suggest caution against climate engineering so as to minimize the (unknown) risks of proposed techniques to the environment and health. On the other, arguments can be made that climate engineering is a precautionary measure against the (known) risks of climate change. This article provides an overview of this debate and what the precautionary principle means in a climate engineering context. It explores, first, how the precautionary principle is interpreted in international law, examining its history, content, legal nature, and operationalization in other areas. Next, the authors consider how the principle can be applied in a climate engineering context, both generally and under existing legal instruments. Finally, the article offers reflections on how the principle can be further operationalized for climate engineering in a meaningful way.