This thesis examines the principle of mutual trust in European Union (EU) law – a nascent legal principle gradually trying to find its place in the EU legal and structural architecture and perhaps even in its constitutional hierarchy. In addition to exploring the principle from a doctrinal standpoint, the work investigates whether the way in which mutual trust has come to function in the Union legal order has the potential to foster actual trust in the EU, and between actors and subjects in the EU legal order. In order to do so, it draws on trust theories proposed by a wide range of social scientists. The doctrinal level of the thesis tracks the historical development and transformation of mutual trust and compartmentalises case law into four distinct jurisprudential generations, each with its specific set of characteristics, terminology, and functions. It does so in a broad manner, touching upon specific policy issues but not constrained by them. In this way, the thesis demonstrates how a principle first deployed in the internal market domain, aiming to encourage Member States to recognise each other’s product inspections not to second guess or duplicate them, became a broad Article 2 value adherence presumption that Member States would live up to ideals such as democracy, rule of law, freedom, and equality, respect for human dignity and human rights. The thesis finds that the prevailing conception of mutual trust as a broad EU law and fundamental rights compliance presumption fails to satisfy even the objective parameters of trust as a concept, meaning that the contexts in which mutual trust is raised are usually not situations which would require an attitude of trust but rather compliance, legality, accountability, and judicial control. The way in which mutual trust has come to operate in the EU is more akin to an undemocratic institutionalisation of trust than the institutionalisation of mistrust the latter being more typical of free and democratic societies. In addition to its critique of the current functioning and understanding of the principle of mutual trust, the thesis offers novel insights toward rethinking and reconstructing the principle in order to better achieve its aspirations.