The EU was not always as big as it is today. When European countries started to cooperate economically in 1951, only Belgium, Germany, France, Italy, Luxembourg and the Netherlands participated.
Over time, more and more countries decided to join. The union reached its current size of 27 member countries with the accession of Romania and Bulgaria on 1 January 2007.
The EU will continue to grow as an increasing number of countries express interest in membership. These countries can be divided into candidate countries and potential candidate countries. The former are in the process of 'transposing' (or integrating) EU legislation into national law, while potential candidate countries do not yet fulfill the requirements for EU membership.