This thesis is concerned with transformations and the different means for implementing territorial engineering. Its aim is to analyse territorial engineering through the emergence of a new instrument, the territorial observatory. It advances the hypothesis that the skills of territorial development professionals play a key role in the effectiveness of this engineering, whereas the way it is structured highly influences the evolution of these skills. We focus on the design and uses of observatories in the Pyrenees Mountains through interviews with the actors who participate in these projects, as well as from the viewpoint of a participating observer within a territorial expertise structure. Observatory projects illustrate the challenge involved in the construction of collective skills for territorial engineering, essential for integrating expertise into the territorial framework. This challenge exists at different levels : between representatives of the same structure and between representatives of different structures. These levels give rise to mediators who encourage the emergence and the consolidation of collective skills. These processes lead to transformations in the individual skills of professionals as well as a crossover between them, thus resulting in the emergence of hybrid professional profiles. In relationship to the overall skills mobilised in territorial development projects, those necessary for territorial observatory projects can be distinguished by a greater need for strategy, while requiring specialised technical skills. The geographic approach to skills as a resource for territorial construction is a major contribution of this thesis.
