The term ‘global catastrophic risk’ lacks a sharp definition. We use it to refer, loosely, to a risk that might have the potential to inflict serious damage to human well-being on a global scale. On this definition, an immensely diverse collection of events could constitute global catastrophes: potential candidates range from volcanic eruptions to pandemic infections, nuclear accidents to worldwide tyrannies, out-of-control scientific experiments to climatic changes, and cosmic hazards to economic collapse. With this in mind, one might well ask, what use is a book on global catastrophic risk? The risks under consideration seem to have little in common, so does ‘global catastrophic risk’ even make sense as a topic? Or is the book that you hold in your hands as illconceived and unfocused a project as a volume on ‘Gardening, Matrix Algebra, and the History of Byzantium’? We are confident that a comprehensive treatment of global catastrophic risk will be at least somewhat more useful and coherent than the above-mentioned imaginary title. We also believe that studying this topic is highly important. Although the risks are of various kinds, they are tied together by many links and commonalities. For example, for many types of destructive events, much of the damage results from second-order impacts on social order; thus the risks of social disruption and collapse are not unrelated to the risks of events such as nuclear terrorism or pandemic disease. Or to take another example, apparently dissimilar events such as large asteroid impacts, volcanic super-eruptions, and nuclear war would all eject massive amounts of soot and aerosols into the atmosphere, with significant effects on global climate. The existence of such causal linkages is one reason why it is can be sensible to study multiple risks together. Another commonality is that many methodological, conceptual, and cultural issues crop up across the range of global catastrophic risks. If our interest lies in such issues, it is often illuminating to study how they play out in different contexts. Conversely, some general insights – for example, into the biases of human risk cognition – can be applied to many different risks and used to improve our assessments across the board.