The work "Solidarity in Union Law" is an attempt to approach a fundamental value principle of union law. The concept of solidarity permeates the entire legal order of the Union, both as a value and as a goal of the European Union. In the Treaty of Lisbon, the principle of solidarity among member states is enshrined in various provisions and is a central axis of important Union policies: from the Area of Freedom, Security and Justice and Economic Policy to Energy policy, European External Relations and Migration.