This thesis considers technological risks and their constitution as public problems. It supports that the main difference among risks is not to see between proved risks and uncertain risks, on the basis of the objective knowledge, but rather separates risks which have already reached compromises at both levels of evaluation and responsibility and those which are still controversial. The starting observation is that social acceptability of risk is out of the scope of the objective approach. Actors use categories of "objective risk" and "perception of risk" within struggles to define what is at stake. It shows how risks, as soon as their evaluation starts, are negotiated within controversies, mobilizing experts, stakeholders, and actors of society. With many environmental risk, and with the globalization of hazards such as climate change, the preventive paradigm is overflowed; and with the emergence of new uncertainties, such as the introduction of genetically modified organisms in agriculture and food, expertise as the only method to deal with risk is contested in society. The precautionary approach, which asks to act against uncertainty before the completion of scientific evidence of risk is declining in the United-States while its integration is increasing in public policy in Europe. Have we entered the risk society? Or, is the society of the developed countries safer than it has ever been? Are risk increasing or decreasing? These apparent paradoxes shows there is different social theories of risk with different possible balance between objective and social factors to explain them. The role of technology is ambivalent, generating new risk and in the same time privileged as a solution to uncertainty. The thesis puts forward "science, technology and society" approach in risk studies, and it aims at a sociology where experts are not considered neutral actors. In this perspective, robustness of expertise does not rely directly on objective knowledge, but on the relation of this knowledge with its context of application. After studying the role of risk in modernization and rationalization processes, the thesis focuses the mobilization of actors in risk controversies and the role of some new institutions in the public sphere such as the ethical committees and the technology assessment agencies. Two case studies are investigated which take place in Switzerland end of the 1990s. The first relates to the introduction of genetically modified organisms in agriculture and environment. A controversy occurred about field test, i.e. scientific experiments refused by the authorities in 1999 and 2001. The role of research in biotechnology is considered as well as the different role of expertise in the public sphere. Expertise in biosafety and social acceptability of biotechnology are negotiated simultaneously. The second case study is the climate change issue and the negotiation of measures to control greenhouse gas. The role of research policy in the governance of climate risk is studied. Both case studies are interested with the role of experts in the public sphere, with the coordination of expertise and decision making, and with the degree of democracy achieved in sociotechnical controversies. The main result of this study is that a linear model of risk dominates approaches and constitutes an obstacle to the democratization of technological change. Instead, the thesis proposes a model where rationality is not defined beforehand, but appears as the proper result of controversies. In this view, risks are socially negotiated, already during the production of expertise, and all along the decision making and the implementation processes.