Belgium, Germany, France, Italy, Luxembourg and the Netherlands were the first Europeans to economically cooperate in 1951. As of 2015, the European Union consists of 28 members with Croatia becoming the newest member in 2013, bringing the total population of the union to more than 500 million.
The United Kingdom, Ireland and Denmark joined the European Union, or EU, in 1973, initiating the first enlargement since the union's creation in the 1950s. Greece joined the EU in 1981, followed by Spain and Portugal in 1986. This time period is dubbed the Mediterranean expansion.
The Soviet Union dissolved in 1991, bring independence to Eastern Europe. This spurred another enlargement of the European Union. Sweden, Finland and Austria joined in 1995. These countries were followed in 2004 by Cyprus, the Czech Republic, Estonia, Hungary and Latvia. Lithuania, Malta, Poland, Slovakia and Slovenia also joined in 2004.
Bulgaria and Romania joined the EU in 2007. Croatia achieved accession to EU membership on July 1, 2013.
Of the 28 EU members, 26 are part of the Schengen Area, allowing international travel without a visa for EU passport holders. The United Kingdom and Ireland are the two countries that have opted out of the Schengen Area. Bulgaria, Croatia, Cyprus and Romania must join the organization by legal obligation.