Who is pushing for a vote to remain "in" the EU?
Most high-level politicians, including Prime Minister David Cameron, Labour leader Jeremy Corbyn, and the leadership of the Liberal Democrats and the Scottish National Party, are supporting the vote to remain in the EU.
Who is pushing for an "out" vote?
Over the past few years, the United Kingdom Independence Party (Ukip) and its idiosyncratic leader, Nigel Farage, have found a modicum of success with an anti-EU. message. While their message was initially somewhat fringe, they have found increasing support from Britain's mainstream Conservative and Labour parties.
Perhaps most notably, a number of top members of the ruling Conservative Government have broken with Cameron on his Brexit policy. Boris Johnson, the colourful former London Mayor, is probably the highest-profile politician to throw his weight behind the "out" campaign.
What does the rest of the world think?
Generally, most foreign leaders have sided with the "in" crowd. Many European leaders, for example, seem concerned about the precedent a Brexit vote might set in the EU. Other foreign leaders, such as US President Barack Obama, have argued that a Brexit would diminish Britain on the world stage and perhaps set off broader chaos.
Obama's intervention in the Brexit debate drew serious anger from some anti-EU-types, with Farage declaring him the "most anti-British American president there has ever been" and Johnson suggesting that Obama's Kenyan heritage made him predisposed to want to see Britain fail.
That's not to say there's no support for a Brexit from international figures. Perhaps more notably, US presidential hopeful Donald Trump has offered pro-Brexit views in interviews, though he has clarified that he was not making recommendations and was just talking personally.
So what would happen if Britain left the EU?
That's a little hard to say: No country as large as Britain has voted to leave the union before. Greenland, a self-governing territory of Denmark, voted to leave the EU's predecessor (the EEC) in 1982, but in very different circumstances.
That, in itself, may be the biggest problem. If Britain left the EU, it would be a sign to other anti-EU groups across Europe that countries can actually exit the union if they want to. That could have a big impact all over the continent, where anti-EU parties such as France's National Front have won significant electoral support in recent years.
By proving that membership in the union is reversible, a Brexit could severely damage the very foundations of the EU, itself a hugely important attempt to create unity on the continent after the ravages of World War II.
For Britain, the economic effects of leaving are a subject of fierce debate. Some Eurosceptics argue that the union will want to maintain good economic relations with Britain, effectively allowing the country to have its cake and eat it. But it's also possible that EU officials would impose tough trade restrictions on Britain as a warning to others that might want to leave.
Additionally, London's status as Europe's financial capital would look shaky if Britain left. HSBC is already warning that it may move a thousand finance jobs from London to France if the Brexit goes through, and a Brexit may well re-spark the campaign for an independent Scotland, because of the pro-EU sentiment held by a large number of Scots.
Why do so many Brits want to leave the EU?
That's another tricky question. Britain has long had an awkward relationship with the EU. The country stalled on joining the European Economic Community (EEC), the precursor to the union, until 1973. Controversy over the terms of its membership led the country to hold a referendum in 1975, which the pro-Europe side won. Britain has avoided becoming too intertwined with some of Europe's institutions, however: It has not switched to the euro, and it is not a part of the Schengen Agreement, which did away with border controls between the countries.
Although it's far from perfect, most analysts argue that union membership has been a boon for Britain, which suggests that, ultimately, the argument may be based more on emotion than economics. Perhaps it's because of the sea that separates Britain from continental Europe, the memories of the British Empire, or the special relationship with the United States, but many in Britain have never really considered themselves a part of Europe. Many Britons hate the idea of giving up sovereignty to a bunch of gray Eurocrats.
Most polls suggest that immigration may be the central issue in the Brexit debate. Many Brits feel that EU migrants who legally move to Britain are taking jobs from local people and abusing the country's benefits system. The EU's troubled response to a recent wave of migrants and refugees from the Middle East and Africa has only made things worse.
