The problem of the situation that has long prevailed in social sciences was based on the idea of “defining the situation”: to act effectively, a rational subject must select and analyse the conditions for its action, interpret its environment and circumstances, and order its impulses and attitudes. Ongoing research in various fields and disciplines (logic, semantic, ecological psychology, artificial intelligence, etc.) shows that such a problem does not reflect the actual logic of situations or the form of control they exercise over experience, and that, in order to grasp both, the analysis of the ecology of social activities needs to be repeated and the impact of the environment on the one hand, on the one hand, on the other, on the other, on the other hand, needs to be better understood. Such requirements already guided the reflection of US pragmatists, J. Dewey and G.H. Mead in particular. They were also the horizon of the work of E. Goffman. The works of these authors therefore contain tools to clarify and deepen the main intuitions guiding current attempts to “locate” activity and cognition, as well as to acclimate them to social sciences. In addition to presenting and discussing these tools, this book strives to test them empirically, on social objects as common as singing improvisation, walking around the city, reception of television or reception and orientation of passengers at SNCF stations.