A crisis in the legitimacy of the State and a crisis in public finances—these phenomena have led to the public and private sectors drawing ever closer together and even overlapping each other’s traditional fields of action. Today, the two sectors collaborate in many ways. This course aims to analyse the economic pertinence of those different collaborative models as well as the advantages and disadvantages that different collaborative tools can offer with regard to the legal system currently in place. Combining economic and legal approaches to this topic is one of the course’s strong points. Free of any ideological references favourable to either more or less State involvement, this course implicitly seeks definitions of better State involvement that can be analysed by economists, jurists, political scientists or experts in administrative science.