As a legal status, Union citizenship involves a certain social element. This cross-border social solidarity component is closely connected to the freedom of movement that represents one of the core rights associated with the EU citizenship status. This chapter looks into the social facet of Union citizenship. It begins by recalling Marshall’s theory of social citizenship and reflects on its applicability in a supranational context. The chapter then proceeds to the analysis of Union citizenship’s social component. This analysis is carried out at three levels that correspond to the residence period of mobile Union citizens: up to three months, from three months to five years, and permanent residence.