While solidarity as an ideal in the legal relationship between a host Member State and the non-national Union citizen has all but vanished from the discourse of EU free movement law, it has resurged in another line of case law concerning Union citizenship. The relationship between the Member States and their own nationals is at the centre of the case law on loss of Union citizenship rights under Article 20 TFEU. The bond of nationality between the individual and the state is there designated as one of ‘solidarity’ and ‘good faith’. This article argues that solidarity, as an ideal, is also relevant for understanding the case law dealing with returning, or naturalising Union citizens who have made use of freedom of movement under Article 21 TFEU. The article provides a discussion on the various expressions of solidarity as a component of the ideal bond of nationality between a Union citizen and their home Member State. Conclusively, it is argued that the meaning of the bond of nationality will continue to develop together with the legal evolution of Union citizenship.