This chapter turns to the third UN mediator in Syria, Staffan de Mistura, and has the same two objectives as the preceding case studies in this book – to delineate the mediator’s agency as a decision-maker and to elucidate the strategic dynamics behind his decision-making. It is split into two sections dedicated to each objective. The first section investigates the mediator’s input to the main mediation policies during the period studied. Of which there are six – the Aleppo Freeze, sanctioning military action against ISIS, the Geneva Consultations, the intra-Syrian Talks, the Astana Process, and the mediator’s resignation. Building on these findings, the section uncovers the dynamics behind the mediator’s decision-making using the categories of perceptions drawn from the contingency model. This analysis also helps produce more generalizable knowledge concerning associations between mediation perceptions and behaviors.