Catastrophic risk raises questions that are not only of practical importance, but also of great philosophical interest, such as how to define 'catastrophe' and what distinguishes catastrophic outcomes from non-catastrophic ones. Catastrophic risk also raises questions about how to rationally respond to such risks. How to rationally respond arguably partly depends on the severity of the uncertainty, for instance, whether quantitative probabilistic information is available, or whether only comparative likelihood information is available, or neither type of information. Finally, catastrophic risk raises important ethical questions about what to do when catastrophe avoidance conflicts with equity promotion.