The term geoengineering lacks a precise definition but is widely held to imply the intentional manipulation of the environment on a global scale. For most of the last 30 years, there has been a wide consensus that such manipulation would be a bad idea. However, in August 2006, Paul Crutzen, the climate scientist and Nobel laureate, published an article that reignited debate about whether we should explore geoengineering “solutions” as a response to the escalating climate-change problem. This was soon followed by other contributions and proposals, and now interest in geoengineering has become widespread, in both academia and the world of policy. As a result, Time magazine recently listed geoengineering as one of its “Ten Ideas That Are Changing the World.” Geoengineering is a relatively new and underexplored topic. This is true both of the science and the ethics. Just as we are not close to fully understanding exactly how to geoengineer if we were to choose to do so, or what the impacts of any geoengineering scheme would be, so we are also not sure how to understand the normative dimensions of undertaking geoengineering. Indeed, at this point almost no moral and political philosophy has even been attempted. In such a setting, it is useful to get some sense of the moral terrain: of what the major issues might be, of how they might be investigated, and so of how understanding might move forward. This is the main aim of this chapter. To pursue it, I shall focus on one prominent argument for geoengineering, raising a number of serious challenges that have wider application. In my view, these challenges are sufficient to seriously threaten the argument, at least in its most prominent and limited form, and so shift the burden of proof back onto proponents of geoengineering. Still, I want to make clear from the outset that my purpose is not to determine whether the pursuit of geoengineering can, in the end, be morally justified.