The Merger Treaty, signed in Brussels on 8 April 1965 and in force since 1 July 1967, first gathered together the organisational structures of the then three European Communities (European Coal and Steel Community, European Economic Community and Euratom).

It created the European Commission and the Council of the European Communities to be the governing bodies for all three institutions, and it also had them share a single budget. This treaty is regarded by some as the real beginning of the modern European Union. The term European Communities or EC also came into use from this time onward.

Timeline of the Treaties and EU Constitution