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The European Union website provides links to all EU treaties, including the founding treaties, the Treaty of Paris (Treaty Establishing the European Coal and Steel Community, 1951), and the Treaty of Rome (Treaty Establishing the European Economic Community, 1957), as well as the more recent Maastricht Treaty (Treaty on European Union, 1992) and the Treaty of Lisbon (2007)
https://europa.eu/european-union/law/treaties_en
The website of the Centre for European Integration Research (EIF) at the University of Vienna publishes working papers on European integration. Its website also provides links to recent publications, useful datasets and the European Integration online Papers archive.
https://eif.univie.ac.at/index.php
The European University Institute (EUI) in Florence is home to the Historical Archives of the European Union (HAEU). On the HAEU website you can find details of its extensive holdings, as well as links to online content and recent relevant publications from EUI researchers.
https://www.eui.eu/Research/HistoricalArchivesOfEU
The Archive of European Integration (AEI) at the University of Pittsburgh is a free, comprehensive electronic repository and archive for research materials on the history of European integration.
The website of the Jean Monnet Foundation contains a range of different written and oral resources including excerpts from Jean Monnet's memoirs describing his role in the process of European integration.
http://www.jean-monnet.ch/
The Virtual Resource Centre for Knowledge about Europe (CVCE) is a resource centre for the study of the EU. It contains links to multimedia resources relating to the history of European integration, including media clips of key historical events such as the Schuman declaration and the signing of the European Coal and Steel Community (ECSC) treaty.
http://www.cvce.eu/home
Between 1998 and 2015, The European Research Paper Archive (ERPA) provided a common access point for a number of online series in the field of European integration research. It has now been archived. Some of the links are now out of date, but it still provides a useful starting point for retrieving research papers on European integration.