British voters will soon head to the polls to make a major political decision about their future — and Europe's. Here's everything you need to know:

What is 'Brexit'?
It's the issue of whether Britain should exit the European Union or not — a question that will be decided in a historic referendum on June 23. Polls show Brits split almost evenly between the "Remain" and "Leave'' camps, with more than 10 percent undecided; passions are running so high that it has divided friends and families. Britain's most influential lawmakers and business leaders have broken into two warring sides, with Prime Minister David Cameron, who wants Britain to remain in the EU, facing a stinging rebellion within his own Conservative Party. Even the royal family has been dragged into the fray, after a tabloid newspaper claimed Queen Elizabeth had privately expressed support for a Brexit. (The royals denied that report.) If Britain leaves, the ramifications for the entire European Union will be enormous, says Guillaume Klossa, founder of the EuropaNova think tank. The recent Greek financial crisis, the Syrian refugee problem, and the terrorist attacks in Paris and Brussels have all combined to raise major doubts about the wisdom of a united Europe, so a Brexit vote could spark a domino effect, says Klossa — jeopardizing "the very survival of the European project."

When did Britain join the EU?
In 1973, though not without much hand-wringing beforehand. As an island that's always had a literal and psychic separation from the rest of Europe, Britain has been a largely unenthusiastic member of the EU. In fact, it has maintained a semidetached relationship, refusing to sign on to the single Euro currency or the Schengen Agreement on passport-free travel throughout the Continent. Nonetheless, "Euroskeptics" from both the United Kingdom Independence Party and Cameron's Conservative Party argue that Britain is too enmeshed with the EU and has gradually ceded control of its laws, borders, and national sovereignty to a burgeoning bureaucracy in Brussels. After coming under great pressure from his own party's Euroskeptics during an election campaign last year, Cameron pledged to hold a decisive In-Out referendum.

What are the pros of a Brexit?
Economically, Britain would immediately save $12 billion a year in EU budget payments. Freed from famously cumbersome EU regulations, Brexit supporters say, Britain would attract greater investment and become a more dynamic economic hub — particularly if it still had full access to the EU's tariff-free single market. But that's a big if, and would rely on Britain renegotiating a new trade deal with the EU's remaining 27 member states — many of whom, post-Brexit, would want to make a bitter example of the U.K., to discourage other members from fleeing.

What about immigration?
This is one of the biggest issues for Euroskeptics. Since it didn't embrace fully open borders with Europe, Britain can already demand that visitors present a passport when entering the country. But under the EU's labor rules, any citizen of a member state has the right to live and work in another member state — a rule that has allowed some 942,000 Eastern Europeans to move to the U.K. as the EU has expanded its borders. Brexiters say these migrants have overwhelmed the housing system and abused Britain's generous in-work benefits. At least 34,000 of them are getting child benefits for children who do not even live in the U.K. and sending that money — totaling about $42 million a year — back to their home countries. Leaving the EU would allow Britain more control over how many migrants are allowed to enter. That's become a big selling point after the influx of 1 million refugees into EU countries.

What would happen if Britain left?
A "Leave" vote would trigger Article 50 of the Lisbon Treaty, starting a step-by-step, two-year withdrawal process. Britain's Parliament would have to repeal all the laws that bind the country to the EU and come up with new trading and travel agreements with the remaining 27 nations. That process "would be a bloody nightmare," says Jonathan Portes, a former chief economist in the Cabinet Office. He warns that the uncertainty it would create could destabilize the markets and cause the pound to plummet. But pro-Brexit campaigner and political economist Ruth Lea says there's no reason to think that Britain, the world's fifth-richest nation, couldn't quickly forge a new and positive alliance with the EU — though "it would take maturity on both sides."

What about the wider consequences?
There could be many. Scotland, which sends half of its international exports to the EU, strongly favors the Remain side, and has threatened to hold a snap referendum enabling it to break away from the U.K. if there is a Brexit. Without a seat at the EU table, Britain might find itself more isolated and its already diminished status as a world power weakened. The EU would lose its second-largest financial contributor, and might find itself crumbling at the edges — particularly if other member states decide to follow Britain's lead. But given that no country has ever left the EU, it's impossible to say with any confidence what might happen. "You could have a reasonably friendly divorce," says French economist Nicholas Veron, "or you could have a very acrimonious one. It's really uncharted territory."

Project Fear
A Brexit will blow a £100 billion hole in Britain's economy, and Britain will lose 3 million jobs! A swarm of migrants is flooding into Britain, and some of them might be terrorists or murderers! As the debate over Brexit gets more fraught every day, campaigners on both sides are using increasingly dramatic doomsday scenarios to try to push the country's undecided voters — about 12 percent of the total — into the "Leave" or "Remain" camps. Known as Project Fear, the cynical tactic is becoming more blatant by the day. Remain campaigners have alleged not only that a Brexit would be an act of "self harm" and would send car companies and banks fleeing abroad but also that it might even cause Brits long-term health problems. The Leave side, in the meantime, has published a "dossier of crimes" committed by 50 EU nationals in Britain, including rape and murder. Whichever side triumphs in June, fear will have played a major role.