In this thesis, I assess the Precautionary Principle (PP) as a guide to decision making in environmental policy. The PP says, roughly, that decision makers should act in advance of scientific certainty to protect against harms to the environment and public health. I argue that the main objections to the PP in the literature can be defended against and, further, that there is a need for such a principle due to the shortcomings of popular quantitative approaches to decision making. I also examine Stephen Gardiner’s influential interpretation of the PP as a version of the maximin decision rule. I argue that Gardiner’s interpretation makes the PP inapplicable in the epistemic situation of ignorance, a situation which most accurately characterizes the nature of our knowledge in many important environmental policy problems. I conclude that the PP should not be interpreted as a version of the maximin rule.