In response to the challenges of policy issues of risk and the environment, a new type of science - "post normal" - is emerging. This is analysed in contrast to traditional problem-solving strategies, including core science, applied science, and professional consultancy. We use the two attributes of systems uncertainties and decision stakes to distinguish among these. Post-normal science is appropriate when either attribute is high; then the traditional methodologies are ineffective. In those circumstances, the quality assurance of scientific inputs to the policy process requires an "extended peer community" consisting of all those with a stake in the dialogue on the issue. Post-normal science can provide a path to the democratization of science, and also a response to the current tendencies to post-modernity.