How organizations cope with institutional complexity remains largely unexplored. Little is known, in particular, on how organizations manage to preserve their identity while responding to the often- conflicting demands of their environment. This issue is even more salient within hybrid organizations, ie. organizations which pursue a social mission through commercial activities, because they have to deal with intrinsic tensions between divergent logics. This dissertation focuses on the case of a French mutual insurance company, the Ma, and on the process of strategic planning it has implemented from 2007 to 2009 to elaborate its orientations. Based on an ethnographic observation of this process and a qualitative analysis at the macro, meso and micro levels, my results show that the Ma is indeed a hybrid and that to evolve and avoid mission drift, it had to overcome its internal tensions and integrate divergent dynamics of “maintenance” and “compliance”. The dissertation further explores the role played by the strategic planning process in this integration. In this case, it was achieved through both the complex shape of the apparatus of strategic planning, which main features were participation and democracy; and the work of internal actors, who demonstrated an ability to seize the strategic issues facing the Ma and to get involved in the drafting of relevant responses. This research aims to contribute to the institutional literature on the organizational responses to institutional complexity and to the "maintenance" of hybrid organizations in particular. It also addresses the literature on the practice of strategy by referring to the role of strategic planning in the integration of organizational tensions. Finally, it sheds light on the issue of democracy and participation in organizations through a tools-based approach, which is rarely used yet highly instructive.