The management of the Eurozone member states' debt crisis, the handling of the refugee crisis, and the Union's response to recent terrorist attacks have brought to the forefront of political and legal discourse the principle of solidarity and its position within the process of European integration generally and within the Union's legal order specifically.

The initiation of this discussion, however, coincided with the entry into force of the Treaty of Lisbon, which for the first time makes extensive use of the term solidarity in the regulatory framework of the Treaties, both in the context of the principles, values, and objectives and within specific policy areas of the Union.

The study initially broadens the conceptual understanding of solidarity within the framework of European integration and its relation to related concepts such as, among others, trust, bona fide cooperation, reciprocity, mutual trust, and responsibility. Subsequently, given the widespread use of the term in the Treaties, it examines the scope of solidarity within the Union's legal order and how it manifests in different policy areas each time.

Finally, the legal nature of the principle is addressed. Does it constitute a fundamental or constitutional principle of European integration, or a general principle of Union law, or merely an ideal with ambiguous and programmatic character?