The thesis investigates the designer s action in conflict and marginalized social contexts aimed at promoting and enhancing local Social Innovation processes through participatory experiences and the Design strategy. Firstly, a theoretical review of the social contribution of Design, of the best-known approaches and practices, and of the selected context was developed. This moment had been preparatory for a subsequent phase of applied research that occurred with the implementation of a participatory Design project inside a Rio de Janeiro slum in collaboration with a local NGO. During field research data were collected through participatory observation. Their later analysis led to identify two key research macro-categories - time, and Design partners and local interests – that were verified and better understood through some interviews with designers and a new phase of theoretical review. All this led to research results: two main issues that may influence, impede or hinder the development of this kind of project were identified. Firstly, time appeared as a fundamental element in project development, able to promote or prevent it. In fact, temporal divergences between the designer and the Design process, between the context and the Design partners may occur. Secondly, the influence of the forces exerted by local actors - and based on interests and inner agendas - on the designer s actions emerged as well as its relation to the project network structure. Research results indicate that: (1) contextual factors may influence the Design process in participatory projects aiming at promoting local Social Innovation processes; (2) the designer s action in participatory experiences may benefit of approaches that consider contextual factors; (3) the methodologies and tools that have been developed to support designer s work in the social field are not enough for action. Finally, this thesis promotes a reflection about the widespread idea of an efficient Social Design practice and about Design education in this area.