The analysis and management of risks in the field of safety, security and environment are complex issues on which the conduct of public expertise is subject to strong stakes. This paper addresses the issues of the understanding, the support and the governance of public expertise process and public decision-making process through research done on risk prevention of major technological accidents. Our onboard research approach has allowed us to see what decision support tools could accompany the expertise process and, thus, the decision-making process. We have shown what it necessary, in areas with strong issues such as risks management, to dispose of methodological approaches to better understand the conditions of expertise and the emergence of recommendations. Opening expertise process to society also allowed us to explore the different modes of participatory governance and propose steps for the design of participatory structures. Different case study (e.g. Seveso high threshold plants, nanotechnologies,...) have enabled us to highlight the different tensions, constraints and biases to which the conduct of expertise is subject.