Mutual trust is a principle of fundamental importance in Union law. It is only because there is a high degree of trust between the Member States of the European Union that the area of freedom, security and justice makes it possible to guarantee fundamental rights, security and access to justice. Although the contours of the principle of mutual trust are uncertain, the Court of Justice has not hesitated to deliver an opinion against the accession of the European Union to the European Convention on Human Rights on the basis, in part, of mutual trust. Opinion 2/13 of the Court of Justice highlights the tensions between mutual trust and the protection of fundamental rights, but also the ambivalent relationship between the two supranational courts. While mutual trust can contribute to better protection of fundamental rights, it may reveal certain divisions. Mutual trust is a tool for integration into the European Union, which is why the challenge of the European Union's accession to the European Convention for the Protection of Human Rights and Fundamental Freedoms must be overcome.