This chapter discusses the role of scientific expertise and under what conditions such expertise can influence decision-makers and enhance the effectiveness of certain environmental regimes. The five regimes studied are whaling, North Sea pollution, acid rain, climate and ozone. All these five bodies have set up scientific advisory bodies. However we find no uniform pattern in political response to scientific advice, but generally advice is most important in the agenda-setting stage and tends to decline over time as policy-makers take control over the process. In general their influenec on decisions and effectiveness is limited unless it interacts with other 'benign' factors of the associated problem-structure. Nevertheless, in three of the regimes, ozone, acid rain and pollution, scientific expertise has played a role in the progress made. Their influence on decisions taken in the whaling - and the climate regimes is less pronounced.