This paper reviews the use of risk assessment techniques in the field of environment protection. I will argue that in some important instances the development of environment policy has been a source of fruitful development of a risk based methodologies. In other cases the importation of risk assessment techniques has proved much more problematic. As the scope of environmental regulation increases so does the possibility of inconsistent and arbitrary solutions to problems. The need for a more systematic approach to the development of environmental regulation has never been stronger, so it is important to understand the reasons for the mixed success of risk assessment. This applies equally to those nations with long traditions of the regulation of private sector industry and those just beginning on this course. The way ahead may be to extend our ideas of how to express risk and uncertainty. Some of the recent cause celebres of environment policy show this challenge very clearly. As an example, this paper will look at the problem of assessing the risk of man-made climate change.