Mutual recognition is the infamous 'cornerstone' principle of the EU criminal law, not to mention of the area of freedom, security and justice of the EU. According to this principle, Member States recognise each other's decisions, judgments and rules. Mutual recognition in turn relies on mutual trust existing between Member States. The Framework Decision on the European Arrest Warrant, the Dublin system are some examples based on such a relationship of trust. However, this premise has not functioned without challenges and problematic presumptions. Violations of fundamental rights, systemic or not, have posed questions about the sustainability of this relationship and they often stress the need for transformation. Nevertheless, the principle of mutual recognition and its prerequisite, mutual trust, have been treated as constitutional principles by the Court. As such, they deserve extensive discussion and in these times of crisis for the EU we must consider the dynamics of sustainability and transformation in this context.