Around 30 million voters are going to the polls in the General Election called by Theresa May just over seven weeks ago.
Polling stations all over the UK are open until 10pm on Thursday, with the results being declared overnight and throughout Friday.
Polling day follows a frantic dash for votes in the final 24 hours of campaigning by the Prime Minister, Labour leader Jeremy Corbyn and the Liberal Democrat leader Tim Farron.
Mrs May campaigned across England in five constituencies; in London, Southampton, Norfolk, Nottinghamshire and Birmingham.
Mr Corbyn made six campaign visits, to Glasgow, Runcorn, Colwyn Bay, Watford and Harrow before ending up in Islington, north London.
Mr Farron also clocked up six visits, to Solihull, St Albans, Twickenham, Carshalton, Bath and Oxford.
But many voters have already cast their vote. It is estimated that by the time the polls close about one in five of the electorate will have cast a postal vote.
This is the first time since 1992 that a General Election has been a stand-alone event without local elections being held on the same day, which should speed up counting on Thursday night.
It is also the first UK General Election where campaigning has been halted by terrorist attacks. It was halted for three days after the Manchester bombing last month and for a day after the London Bridge attack.
The snap poll was called by the Prime Minister in a shock announcement on 18 April, the day MPs returned to Westminster after Easter.
Some 650 seats in the House of Commons are being contested, 553 in England, 59 in Scotland, 40 in Wales and 18 in Northern Ireland. To win a overall Commons majority, a party needs to win 326 seats.
At the last General Election in 2015, David Cameron won a majority of 12 for the Conservatives after five years leading a Coalition government with the Liberal Democrats. Turnout in 2015 was 66.2%.
The Conservatives won 37.7% of the votes and 331 seats, Labour 31.2% and 232 seats, UKIP 12.9% and one seat and the Liberal Democrats 8.1% and eight seats.
But Mr Cameron then quit as Prime Minister immediately after the UK voted to leave the European Union in a referendum on 23 June last year and he was succeeded by Mrs May last July.
In this election there are fewer candidates, 3,306, than in any General Election since 1992, largely because UKIP and the Greens have stood aside in certain constituencies.
UKIP is fighting 378 seats, 246 fewer than in 2015, and the Greens 468, 105 fewer than two years ago.