This chapter addresses the precautionary principle, a principle pervasive in environmental decision-making in many legal systems. The chapter discusses the sources of uncertainty that give rise to the need for precaution and explores why risk regulation and cost-benefit analysis cannot fully address the range of sources of uncertainty for decision-makers. Yet the principle has come under attack because its open-endedness leads to fears that it allows unfettered decision-making that could be anti-scientific and over-reaching. The chapter explores the puzzles of interpretation that give rise to these fears, namely whether the principle should be treated as binding or non-binding, whether it should require some kind of trigger or threshold of risk before it can be invoked, what implications the principle has for the burden of proof for decision-makers, and whether it is temporary or indefinite in its application. It also explores the way in which institutional context shapes the application of the principle. The chapter finds that the way in which these puzzles of interpretation have been resolved in practice demonstrates that fears that the principle has or will lead to unfettered decision-making are unfounded. They also demonstrate that the principle has been applied in relatively modest ways that may not fully engage with the uncertainty that gives rise to the need for the principle.