This thesis presents analyses of the public involvement in decision making processes in local government, and of the mobilisation of scientific expertise by the public engaged in these processes. The theoretical perspective of this research is at the intersection of Science Studies and Political Sciences. The Science Studies perspective is, however, clearly predominant. This research, based on semi-open interviews, is empirical and the cases selected are decision-making processes with a strong technical-scientific dimension. More precisely, the cases selected are ten recent decision-making processes for the setting-up of incineration plants in France: the decision-making processes selected ended between 2003 and 2005. Beyond the empirical research, the research intends to improve existing theories and to provide novel concepts for the analysis of the public engagement and the mobilisation of scientific expertise by the public. Part (i) shows that, in spite of the numerous legal possibilities that the local public authorities have, the level of public engagement was low. There was therefore very little political will to engage the public. The analysis suggests that an early participation sponsored by the public authority may diminish the degree of controversy of a decision-making process, whereas consultation is less likely to reach this goal. Finally, it seems that a high level of communication initiatives sponsored by the local NGOs is likely to lead to the abandonment of the incineration plant project. Part (ii) shows that local NGOs may make positive contributions to public decisionmakings, widening the issue under discussion, and bringing alternative valid scientific and technical expertises. Therefore, the analysis support the position hold by many Science Studies scholars who claim that public participation is likely to improve the quality of the overall expertise delivered to decision-makers. The analysis is also an agreement with a fairly positive view of NIMBY, which supports that the public may have a good grasp of and reasonable concern for health and welfare which are ignored by technical and administrative elites. Finally, the analysis supports the position hold by the ‘critical’ Public Understanding of Science scholars, that is, that the public is able to reflect on the source of their knowledge.