In its case law on the Area of Freedom, Security and Justice, the ECJ has put great emphasis on the “principle of mutual trust”. However, there is still relatively little analysis of the nature and content of the principle as it applies across the whole range of AFSJ law. This article aims to provide such an analysis. Taking a holistic approach, it examines the principle as established under Treaty provisions, legislation and case law relating to the AFSJ. It addresses its legal effects and unpacks its core normative propositions, also revisiting the issue of its relations with fundamental rights. The article concludes that, in essence, the principle is no more and no less than a bundle of interpretive doctrines. Its function is to enhance the effet utile of AFSJ legislation and to provide it with some doctrinal unity, without constraining the legislature's wide discretion in determining the form or degree of “mutual trust” required of the Member States under every individual AFSJ instrument.