The Treaty on the Functioning of the European Union (TFEU) entails the right:
For an explanation of European citizenship relating to Articles 20-25 of the Treaty on the Functioning of the European Union, visit the website of DG Justice in the Commission, which provides links to the triannual citizenship reports.
Read also the EU Citizenship Report 2017 "Strengthening Citizens’ Rights in a Union of Democratic Change".
For issues relating to citizenship more generally across the world, the GLOBALCIT website is indispensable. It has comparative information on the acquisition and loss of citizenship, a section on European citizenship with an extensive bibliography and runs debates on topical EU themes.
CEPS is a very good source on migration, data privacy and mobility.
More resources on:
ECIT’s work focuses on how to close the gap between the fine principles of European law and actual practice on the ground, and barriers to the practice of European citizenship, particularly for job-seekers and vulnerable minorities. The ECIT Guidelines call for a one-stop shop, prevention, collective action and more rapid enforcement. A European free movement solidarity fund is proposed. A rights based European citizenship will not though be complete without full political rights, equal treatment and access to this status for legally resident third country nationals.