The purpose of the following study is to analyze the interdependence between the principle of sincere cooperation between the Member States of the European Union and the A1 certificate. First, the author analyzes the theoretical assumptions for the A1 certificate, especially in the context of the title principle. The document in question is not only a guarantor of fundamental freedoms, but also a sign of trust between the Member States. The author poses questions and considers numerous doubts regarding the issue of trust between the Member States, the limits of this trust and possible control measures, especially in the context of the abuse of treaty freedoms.