In this thesis, in the light of the integration theories, the enlargement of the European Union is described with a historical perspective comprising the enlargement process in the 1990s. A special focus is given to EU - Turkey relations continuing over four decades through the perspective of the EU enlargement. The EU has been inclined to enlarge ever since it was founded. Actually, enlargement has been both a cause and an effect of the many different policies of the Union. Although enlargement became an agreed fact of the Union by more than doubling its size through four different enlargements, it has always initiated some debates within the Union whenever it has become palpable. With the changing international as well as continental conditions of the 1990s, Europe has witnessed the EU’s growing role as the core organisation for conducting the Central and East European Countries’ ‘return’ to Europe. These circumstances have forced the EU to change its ‘classical’ method of enlargement to a more ‘adaptive’ method by challenging the structures of the EU, and forcing the Union to adapt its system accordingly. These conditions also encouraged Turkey in its quest for membership to the EU. Although the integration theories prove insufficient to explain the complete dynamics of the enlargement process, enlargement is an agreed and continuous policy which will prevail the agenda of the Union in the near future. It is concluded that the EU is predicted to start its fifth enlargement by the year 2003.