Voters across the U.K. cast their ballots in a referendum Thursday to decide whether to remain in the European Union. The so-called Brexit campaign has seen plenty of mudslinging and vitriolic statements. Here’s a look at why the economic and political bloc has prompted so much debate in the U.K. and why other EU members are also examining their membership.

What is the European Union and when was it formed? The European Union is an economic and political bloc that today has 28 members stretching from Britain east to Poland. The EU was formed out of the desire to ensure peace and economic prosperity on the continent after World War II and to end “the frequent and bloody wars between neighbors.” Six founding states — Belgium, France, Germany, Italy, Luxembourg and the Netherlands — established the European Coal and Steel Community in 1951 to put their heavy industries under common management to ensure no one country individually could make weapons. In 1957, the Treaty of Rome created the European Economic Community, known as the common market.

When did the U.K. join the EU? The EU began expanding in waves and the U.K. joined in 1973 along with Denmark and Ireland. British voters were asked to decide in 1975 whether they wanted to continue their membership in the European Economic Community and more than 67 percent voted to stay.

Brexit Notices are displayed outside a polling station in central London, June 23, 2016. Photo: Reuters/Stefan Wermuth

What are people’s problems with the EU? The “leave” campaign has argued the EU has grown into a much different entity than what the U.K. joined in 1973. Pro-Brexit supporters argue Britain outside of the EU would regain its sovereignty, would be free from numerous regulations coming out of Brussels and would be able to control immigration. The “leave” campaign has touted the statistic that the U.K. sends 350 million pounds ($520 million) a week to the EU. The correct figure is much lower when trade and flows back and forth are taken into account, the Guardian reported.

Do other countries still want to join the EU? Yes. Croatia became the EU’s newest member in 2013. There is a large group of candidate countries and potential candidate countries that would like to join, including Albania, Bosnia and Herzegovina, Macedonia, Kosovo, Montenegro and Turkey. Ukraine’s government has also expressed its desire to join the bloc.

What other EU members want referendums? While many eastern and southeastern European nations want to join the EU, Britain’s referendum Thursday has inspired other members to consider their relations with the economic bloc. Italy’s 5 Star antiestablishment movement, which recently won the mayoral race in Rome, has called for a referendum on whether to leave the EU and get rid of a single currency system. The EU uses the euro as its bloc-wide currency (Britain, unlike Italy, always kept the pound).

“The euro as it exists today doesn’t work and we need to consider other alternatives such as a euro 2 or alternative currencies,” said party leader Luigi Di Maio, the Wall Street Journal reported.

Marine Le Pen, France’s far-right National Front party leader, said if she is elected president next year, she would call for a referendum, or a Frexit. “France has perhaps a thousand more reasons to leave the EU than the English,” she said.

The Brexit vote has fueled fears a “leave” vote could push EU members Sweden and Greece also to hold referendums.