The polls are open and millions of people around the country will be turning out to cast their ballots before polling stations close at 10pm
But when will we know the result?
Sunderland is expected to once again declare first, sometime between midnight and 1am, with a flurry of results coming in between 2am and 5am.
You can follow our live referendum blog here to keep up with results
Votes are being counted broadly along council boundaries, with 382 separate counts across the UK and 32 in Scotland. Once those local results are reported, they will feed into 12 regional and national announcements. The North East's results are being coordinated in Sunderland.
But it's going to be harder to predict a result based on local counts than it is in general elections - we can't measure the "swing" against a previous result, and people are not necessarily voting along party lines.
There is a huge amount of survey data available, and number-crunchers have created models of how each area would be expected to vote. But it's a tough to call.
Broadcasters have crunched numbers from a huge amount of survey data to create models of how each area would be expected to vote if the national result was a dead heat.
“Scotland and London are expected to vote to remain, come what may,” says John Curtice, the president of the British Polling Council.
“If Scotland is close to 50/50, you can go to bed because we’re out of the EU. The same is true of London.”
In the rest of England, the key areas for Remain are large urban centres, but also smaller cities with large populations of students and graduates.
Leave will be looking closely at results from areas with older populations and lower levels of educational attainment, particularly in coastal east and south-east England.
“To be honest, you don’t have to know much more than where Ukip has done well. If they vote to remain, it’s all over,” says Curtice. Boston, where a result is expected at 3am, will be top of that list.
As local tallies come in, there should be an indication of “which way the wind is blowing,” according to Curtice.
But, he warns, there are many parts of the country “where you would expect the result to be close to 50/50”, so it will be calculators at the ready since the only real indicator will be the total vote number of votes for Leave and Remain.
What that means is that depending on how close the result is, David Cameron is expected to make a statement on the steps of Downing Street in time for the breakfast news bulletins.
What he will say is, until tomorrow morning, anyone’s guess.