Zasada solidarności między państwami członkowskimi w prawie Unii Europejskiej – zmierzch czy renesans?

In recent years, in reference to what has been called the “2015 migration crisis”, in EU Member States and in the forum of its institutions a lot is said about solidarity in the Union, both as a value and as a legal principle. Solidarity and, consequently, the entire European integration project is also said to be “in a state of crisis”. The aim of the article is to answer the question whether we are really dealing with the twilight of the legal principle of solidarity between the Member States of the Union, or, on the contrary, its renaissance. Therefore, the issues discussed include: the qualification of solidarity in the legal system of the Union (relations between value and principle in Treaty terms, way of expressing the principle in the Treaties), understanding of the principle of solidarity between Member States in doctrine and judicature and the role of this principle in the jurisprudence of EU courts as well as its effectiveness and perspectives (observance by the Member States, place in the reflection on the future of the Union, the concept of flexible solidarity). The hypothesis is that - contrary to popular belief - the principle of solidarity between Member States has now entered into one of the best periods in its history as the values and principles of the legal system of the European Communities and the European Union. The paper makes use of theoretical and dogmatic research methods. The result is the conclusion that solidarity between Member States remains the central principle of the legal and institutional system of the Union, without losing its strength of political influence and, above all, the capacity to produce (indirectly) legal effects. The migration crisis and the attitude of some Member States have contributed to the flourishing of the principle of solidarity as an object of reflection on the law and the future of the Union.