Mutual trust is the basis on which the Member States’ judiciaries are expected to deal with each other in the European Union. By constitutionalizing the principle of mutual trust, the CJEU has introduced an axiological addition to the basic structure of the European Union. From a Union which concentrated on the vertical relationships between each Member State and the central Union’s institutions, the Union has turned out to be additionally preoccupied with the horizontal relationships among the Member States, which are based on what might be called a doctrine of Horizontal Solange.According to the principle of mutual trust, each Member State must presume that all other Member States are in compliance with EU law, in particular promote its values and respect European fundamental rights. This presumption, however, can be rebutted in exceptional circumstances. These exceptional circumstances are based on a two-prong test: first, the violation of the values or the fundamental rights must amount to a systemic deficiency; second, there is a need for an assessment whether the individual concerned will be the victim of this systemic deficiency.This contribution critically analyses these exceptional circumstances. Regarding the first prong, it is argued that the existence of systemic deficiencies should ideally be established by the CJEU via preliminary ruling references or via direct infringement proceedings. Alternatively, such systemic deficiencies may also be established by domestic courts in a host Member State. Regarding the second prong, it is argued that the individual test is redundant in cases where the systemic deficiency imposes challenges to the existing legal order of the Member State in question. Finally, it is argued that the suspension of mutual trust can serve as a decentralized instrument for protecting the European rule of law by pressuring the violating state to restore the rule of law.