Chapter 10. Environmental Risk and Uncertainty Environmental risks may comprise the most important policy related application of the economics of risk and uncertainty. Many biases in risk assessment and regulation such as the conservatism bias in risk assessment and the stringent regulation of synthetic chemicals reflect a form of ambiguity aversion. Nevertheless there is evidence that people can learn from warnings and risk information such as Toxics Release Inventory data consistent with Bayesian models. The fundamental uncertainties with respect to environmental risks are coupled with irreversibilities making sequential decisions and adaptive behavior desirable. Uncertainties over the benefits and costs of mitigating environmental risks pose challenges for any regulator but insights drawn from the instrument choice literature can inform the design and implementation of welfare maximizing environmental pollution control policies. The problem of mitigating climate change risks motivates a series of illustrations of how uncertainty affects policy.