The purpose of this article is to try to outline the essence of membership of the European Union. This international organization, by virtue of the decision of its creators, i.e. the Member States, has been equipped with attributes, which have determined its unique – supranational – character. As a new legal order, the European Union has been granted some scope of autonomy, but ontologically it is dependent on the Member States. It is the Member States that have taken decision on setting up a new integration structure with a center of decision-making located not only outside but also above them, the scope of its competences and instruments of their exercising, and as “masters of the Treaties”, may decide to dissolve it. The decision to join the European Union seems to be determined pragmatically and praxiologically – upon benefits of cooperation within the framework of the EU. In this perspective solidarity, understood as the unity and equality of the Member States, based on common values, becomes a factor legitimizing the EU, and at the same time – a guarantor of its existence, especially in times of crisis.