The dissertation is about scientific uncertainty in the policy process, that is when scientific knowledge about an issue is lacking or the existing knowledge is uncertain. The aim of the study is to understand if and how scientific uncertainty affects the policy process. For that purpose, the development of climate change policy in Sweden is studied, from 1975 until 2007. The material studied consists of interviews with politicians, bureaucrats and scientists, newspaper articles and debate articles, as well as governmental and agency material. The theoretical framework developed and used in the dissertation builds on John Kingdon’s multiple streams framework as well as insights from Science and Technology Studies, relating to the production of knowledge and the relation between scientists and society. The study shows that scientific uncertainty is only one among many factors that affect the policy process and that it has a limited influence. The area where scientific uncertainty, in the case studied, has had most influence is in the selection and formulation of policies. There policy makers have used a number of ways to make decisions in the face of uncertainty. These include reframing the issue so that uncertainties are no longer relevant, relying on the precautionary principle, requesting more research, and basing decisions on the judgment of scientists. The most common way of managing uncertainty has been to reframe the issue. In the case studied, scientific uncertainty has made scientists very influential as to how climate change has come to be understood as a political problem. Yet they have had a very limited influence over the formulation of climate policy