To become the next president of the United States, President Donald Trump or former vice-president Joe Biden will have to win at least 270 electoral votes out of a total of 538, whether or not they win the nationwide popular vote.
Most states have a history of voting Democrat or Republican, so the election will come down to a few crucial states that could swing either way.
Nov 4, 8am-11am Singapore time: Which way will toss-up states fall?
Polling will close in Florida, Georgia, Texas, North Carolina and Ohio between 7pm and 7.30pm (8am and 8.30am Singapore time).
In Iowa, polling stations will close at 9pm (11am Singapore time).
Mr Trump and Mr Biden have a 50-50 chance of winning most of these states, even though the President won them by comfortable margins in 2016.
No Republican president has won the White House without Florida - which has 29 electoral votes - in 100 years, and losing Florida will be very hard for Mr Trump to come back from.
Democrats believe that even Georgia (16 electoral votes) and Texas (38), which lean Republican, are in play for them this election.
If Mr Biden flips states like Georgia, Iowa (six), Ohio (18) and Texas, he may be headed for a decisive victory.
9am-10am: Will the Midwest keep faith with Trump?
Polls close in Michigan and Pennsylvania at 8pm (9am in Singapore), while in Wisconsin, polls close at 8pm (10am Singapore time).
Mr Trump famously won the three "blue wall" states of Michigan (16 electoral votes), Pennsylvania (20) and Wisconsin (10) by 77,000 votes in total in 2016, dashing the presidential hopes of Democratic candidate Hillary Clinton.
If Mr Biden flips Florida blue, a victory in one or more of these states could seal the presidency for him. Winning all three of these Midwestern states could also be a path to the White House for Mr Biden.
If Mr Trump holds on to them or the contest remains close, prepare for a long-drawn-out night.
10am-11am: Will they stay with Biden?
At 7pm (11am Singapore time), polls close in Arizona and Nevada, while in Minnesota, they will close at 8pm (10am Singapore time).
The two Sun Belt states of Arizona (11 electoral votes) and Nevada (six) lean Democrat at the moment, but the Trump campaign is gunning for them. Minnesota (10) has voted Democrat since 1972, but Mrs Clinton won it by a narrow 1.5-point margin in 2016.
If Mr Trump wins these states, or comes close to winning them, it may signal trouble for Mr Biden's campaign.
However, RealClearPolitics' polling average as at Nov 2 showed that Mr Biden is leading in the three states by between 0.9 and 4.3 percentage points.
From 11am: Is the contest close?
Ordinarily, major TV networks and news outlets will call the race based on exit polls before an official announcement is made. The winner has been called as early as 9am, although in 2004, 2008 and 2012, this happened at around or just past noon. In 2016, Mrs Clinton called Mr Trump to concede at around 3.45pm.
This year, experts say it can take days or even weeks to count all the ballots. Processing and counting mail-in ballots take time, and some states cannot start until after polls close. Other states accept ballots after Nov 3, as long as they are postmarked on election day. Absentee voting rules may be contested in court, lengthening the wait before the winner is finalised.
If either candidate appears headed for a big victory, the chances that the result will be disputed will be lower.
But if the contest is close, and if ballots counted later swing the race in favour of the Democrats, this could prompt calls from Mr Trump that the election is rigged.
The President has also declined to say that he will accept the results of the election, leaving a question mark hanging over how election night will turn out.
For live updates and results, follow our US election live coverage.
Sources: Cook Political Report ratings