After a bitter and divisive campaign, we should know who will be the next US president later today.
Millions of votes had been cast before today as 38 of the 50 states had some form of early voting in this year's race to the White House.
But millions more voted today and lengthy queues were seen at many polling stations.
Unsure about the US election? @BrianOD_News explains electoral colleges, swing states and what to look out for https://t.co/FdYBYvNN4S
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Projections from each state will be available on a rolling basis as polls close across the country; the first polls are set to close at midnight Irish time.
Bellwether states such as Ohio might give an indication of how the result is going. Ohio closes polls at 12.30am Wednesday with projections due shortly afterwards.
The election could be called by US TV networks as early as 4am Irish time on Wednesday. Voting will have closed in all states by that time, except Alaska.
Democratic presidential nominee Hillary Clinton, who is hoping to become the first woman president of the US, will hold her election night rally underneath a glass ceiling in a Manhattan convention centre.
Republican Donald Trump, hoping to become Barack Obama's successor, is to hold his election night "victory party" at the New York Hilton Midtown.
Below is our guide on the states to watch - the battleground and possible battleground states.
The drama begins at midnight Irish-time when polling stations close in Georgia, South Carolina, Vermont, Indiana and Kentucky.
The first surprises could come from Georgia if Trump loses that normally Republican-leaning state, and if Virginia, won by Barack Obama in 2012, eludes Clinton.
Half an hour later, the surprise could come from the battleground states of Ohio and North Carolina, which have 18 and 15 electoral votes, respectively.
Ohio, a fallen industrial powerhouse, is historically Democratic territory while North Carolina tends to vote Republican. But either could swing the other way this year.
Over the next 90 minutes or so, a burst of results from some 30 states accounting for dozens of electoral votes will further fill in the red and blue electoral map.
All eyes will be on Florida - an ethnically and politically mixed snapshot of the US that accounts for a whopping 29 electoral votes.
Obama won it narrowly in 2012. Back in 2000, in the historic disputed election that kept the nation on the edge of its seat for days, George W Bush defeated Al Gore there.
Much attention will also be paid to the small northeastern state of New Hampshire, which normally votes for Democrats but is seen as possibly up for grabs this time, and Pennsylvania, another former industrial powerhouse that normally leans Democratic and holds a prize of 20 electoral votes.
Heading west, Arizona and Texas - conservative states that border Mexico - are seen as possible prey for Clinton.
And in the other direction, Colorado, Michigan and Wisconsin - Democratic strongholds that account for 35 electoral votes between them - are seen as maybe going to Trump.
Americans are also voting for every one of the 435 members of the House of Representatives, who serve two-year terms, and for 34 of the 100 senators, who serve for six years.
They are also choosing governors in 12 states, and will be casting votes in a multitude of referendums and local elections.
Full coverage | Electoral College Explained | What the candidates must do to win
Follow RTÉ's team in the US: RTÉ's Washington Correspondent Caitríona Perry | Richard Downes | Rachael English | Cian McCormack | Sean Mac an tSithigh | Fergal Keane | and RTÉ News
Follow the results live
RTÉ will provide extensive coverage on results night, across online, mobile, television and radio.
RTÉ.ie/news and the RTÉ News Now app will bring live results through the night.
RTÉ One television will broadcast a US Presidential Election Special programme from 11.25pm on Tuesday through the night until 3.30am.
Live television coverage will continue with the simulcast of CBS News: US Presidential Election coverage from 3.30am until 7am on RTÉ One.
Beginning at 10pm on RTÉ Radio 1, an extended Late Debate with Cormac Ó hEadhra will invite indications and exit poll discussion from panellists.
At midnight, RTÉ Radio 1 will broadcast live election coverage from NPR, America's public service broadcaster, to keep listeners up-to-the-minute through the night.
From 6am Wednesday, a special Morning Ireland programme from Washington DC and Dublin will bring the latest news and reaction.
At 9am on RTÉ One, Sharon Ní Bheoláin will present the latest news on the results in an hour-long US Presidential Election Special programme.
The 45th President of the United States will be sworn in to office on 20 January 2017.
He or she will take an oath on the steps of the US Capitol building in Washington DC and take office at noon that day.
