This document is an excerpt from the EUR-Lex website
Deepening and widening are 2 schools of thought as to how the EU should develop.
Over time, the European Union has progressively evolved into what aspires to be an ‘ever closer union’ among the peoples of Europe (Article 1 of the Treaty on European Union - TEU).
The notion of deepening refers to this ever closer union and is seen in the increased integration of the EU. Its clearest manifestation has been the EU's transition towards economic and monetary union (EMU) and the introduction of the single currency, the euro.
Proponents of widening consider that the EU should expand in terms of membership but that this membership should be looser than that desired by the deepening school. The EU has also managed to widen itself, enlarging from 15 countries in 2004 to 28 in 2013.