Research on the reliability of high-risk systems shows that there are failures in organisation which arise from the difficulties that engineers and technicians have in conceiving organisation in relation to the technology used. This thesis treats the design process as a social, contingent and contextual construction. An anlysis of the contribution of actors engaged in the modification of high-risk chemical installations shows that their anticipation of the organisation in relation to the technical systems is partial. The social and dynamic perspective adopted reveals the strenghts and weaknesses of organisation and design practices in order to take account of the major professional risks. Moreover it questions the human and socio-economic costs of design in highly regulated industries where demands of profitability and competitiveness are additional to the requirements for risks management.