Building on discourses on trust developed by other social sciences, this collective working paper aims to frame the meaning and reach of the notion of mutual trust as relied upon in the field of EU law with a view to contributing to its conceptualization. The structure of the paper reflects that ambition: starting with a rich contribution replacing mutual trust within the context of the history of ideas, followed by a critical reflection on the role of mutual trust in the EU market integration process, the working paper then carefully inquires into the role of mutual trust in the Area of Freedom, Security and Justice - in particular in the fields of civil and criminal justice cooperation - where references to mutual trust have been the most apparent over the past decade, before attempting to understand the significance of mutual trust for the management of the Union as a polity. Albeit to varying extent, all legal contributions explore the potential and limits of mutual trust as a notion governing regulatory choices and judicial interpretations, if not the EU legal system as a whole, while being anchored in substantive law analyses. All contributions were finalized at the end of 2015.