Since its formation in 1950s as the economic community, the EU has created the monetary union and is increasingly evolving also into a political union – part of which is also a union or Europe of citizens. This article explores the development and the existing EU legislation and case law of the Court of Justice of the EU (CJEU) on Union citizenship. The article emphasises the importance of the case law of the CJEU for the development of this concept, focusing especially on the case law pertaining to access to social security benefits in another Member State, the rights of students, tax relief, and personal rights such as the right to write a name in a certain way and the right to family life. Case law of the CJEU has, inter alia, confirmed that even economically inactive Union citizens lawfully residing in another Member State have a right to access to social benefits under the same conditions as the Member State’s own nationals. The concept of the Union citizenship is of key importance in the development of EU law, as it expands the scope of the applicability of the provisions on free movement of persons and other fundamental freedoms. New challenges and questions linked to Union citizenship are arising over time, which should also be regulated at the EU level in the future. Therefore, further development of this concept can still be expected in the EU.