Research into the inter-organizational relationship (IOR) is proliferating. In several social science disciplines, such as economy, strategy, organizational and management research the IOR has become a topic of substantial importance. Accounting researchers have been slow to explore the control mechanisms within IOR but, for some years now, have started to give some attention to it. After introducing the recent works on the subject, this study applies a framework of concrete control modes to the French public-private partnership model: the concession contract. Drawing from a case study, it highlights the complementarities between formal and informal controls. It also clearly shows the nonlinearity of the link between trust and controls.