This paper will discuss the concepts of resilience and social-ecological system, applied to Sahelian pastoralist's societies, proceeding with an analysis of pastoral mobility as a "total social fact". Sahelian pastoralists live and keep their herds at multiple time scales : seasonal and intra-seasonal pasture prospection, weekly rotation of markets, religious slaughter based on lunar calendar, heavy annual rainfall variability (30% on average), historical transformation of the territories they use, multi-year drought, and the decennial history of development interventions, from state enterprises to local and international projects, through decentralized cooperation. The flexibility of their mobility patterns and systems of activities allowed pastoralists to resist and cope with evolving changes, constraints and uncertainties. Access to different and complementary ecosystems guarantees "functional integrity" to pastoral systems as well as the plasticity of the activities enables pastoral societies to withstand crisis relatively better than other rural professional groups. Mobility is not just technical, it's multidimensional (zoo-technical, economic, social, political, cultural), it's a "total social fact". Science sometimes tends to simplify this complexity : thus, the concept of socio-ecological system, assuming a social group confined in "his" ecosystem, is challenged by the pastoral mobility, which is based on the complementarities and rhythms of areas. In the same way, the notion of resilience, supposed to describe the capacity of systems to adapt and persist on the long term, paradoxically fail to analyze processes of change. In northern Senegal, pastoral systems have experienced deep social, territorial and environmental changes. Behind the "resilience of pastoral systems," the pastoralists themselves, their social relations related to land and livestock, their technical and economic issues and even their cattle have changed. What then could be the scope of th