The purpose of the article is to discuss the importance of mutual trust between EU Member states, which is key to the process of European integration. Without mutual trust it would be impossible to create an Area of Freedom, Security and Justice (AFSJ), which in this aspect applies to both the legal systems and the justice systems of the Member States. Mutual trust allows to assume that the legal system of each Member State guarantees the same level of protection of individuals and that law enforcement bodies carry out their tasks equally effectively. However, the level of mutual trust is not constant - one can observe its slow and systematic growth, but its level may as well decrease, which has recently occurred in the case of relations between Poland and other Member States. This condition has an adverse effect on the functioning of the AFSJ and causes the occurrence of many practical problems, which are presented in the article. The legal order was determined based on the formal-dogmatic method and presented using the descriptive method. The case study method was used when presenting CJEU rulings.