Discussions of climate change and climate change mitigation frequently mention the Precautionary Principle—the idea that even if the normal scientific standards for establishing cause-and-effect connections are not met in the case of the relationship between some industrial or technological activity and harms to human health or the environment, precaution can warrant the regulation of that activity, especially if the harms are catastrophic or irreversible. This “better safe than sorry” approach is supposed to run counter to the standard regulatory decision procedure of cost-benefit analysis, in which possible but unproven causal connections are significantly discounted or do not count at all. In this chapter, the Precautionary Principle is located within the broader philosophical discussions of both environmental ethics and of risk and decision-making. The history of the Precautionary Principle is also surveyed briefly. A core common structure of the Precautionary Principle is identified, and key component concepts are distinguished. Several criticisms of the Precautionary Principle are advanced and responses to those criticisms presented. Finally, the relevance of the Precautionary Principle to climate change mitigation is discussed briefly.