This Ph. D thesis is dedicated to a specific type of expertise, the safety of nuclear installations in the field of human and organisational factors. Empirical work is at the foundation of this thesis: the monitoring of experts “in action”, allowed a detailed reconstruction of three cases they were examining. The analysis, at the core of which lies the definition of what an efficient expertise can be, leads us to identify the expert’s three ranges of actions (rhetorical, cognitive, operative). Defined from objectives and constraints likely to influence the expert's behaviour, those three ranges each require specific skills. A conception of expertise based on these ranges seems adaptable to other sectors and allows an enrichment of models of expertise cited in literature. Historical elements from French institutions of nuclear safety are also called upon to take into consideration some of the determinants of the expertise; its efficiency relies on the upholding of a continuous dialogue between the regulators (the experts and the control authority) and the regulated (the operators). This type of historically inherited regulation makes up a specificity of the French system of external control of nuclear risks.