Greenhouse gas emissions abatement, negative emissions technologies, and adaptation are not, and most likely will not, be enough to prevent dangerous climate change and its deleterious impacts on humans, other species, and ecosystems. Some scientists and others are increasingly considering and researching solar geoengineering, which would reflect or block some of the sun's incoming solar radiation, as a potential complementary response. This introductory chapter offers an initial explanation of climate change and solar geoengineering, including its leading proposed techniques of stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning. Solar geoengineering should be taken seriously, as its governance is both important and challenging. Among the major challenges is that solar geoengineering presents a high-stakes risk-risk tradeoff under conditions of great uncertainty. Another is that although earlier governance can be more effective, little is then known of such an emerging technology’s salient characteristics. The chapter outlines the topics covered by the remainder of the book and makes the author’s prior assumptions explicit.