As the most divisive election in US history draws to a close, 130 million Americans are expected to vote today across 50 states.
However, the complicated electoral system means that winning the popular vote will not necessarily lead to victory.
The 538 votes in the electoral college mean the eventual winner must secure a total of 270 or more to secure victory.
There are 115 electoral colleges currently on a solid Democrat vote and 88 looking likely to vote Clinton.
Trump can almost certainly rely on 63 electoral college votes from red states with 101 now leaning his way.
That leaves 171 swing electoral college votes which will be closely watched tonight.
The crucial states in question are Ohio, Florida, Colorado, Iowa, Virginia, North Carolina, Nevada, Pennsylvania, New Hampshire and Arizona.
Arizona has been traditionally Republican - but could change for this election.
Trump must win in Florida and Ohio or victory will slip away from him.
Meanwhile, Clinton was forced to adopt a defensive stance in the late days of the campaign following the stepping up of the FBI investigation into her email use of a non-approved server.
Clinton spent her last few campaigning days shoring up the Democrat states of Michigan and Pennsylvania. Falling short in these traditional Democrat heartlands could prove fatal to her ambitions.
Clinton spent the very last night on the hustings with her family and the Obamas in state capital Philadelphia, so a loss here would be a bitter blow.
Key factors will be the size of the turnout among Hispanic and African-American voters.
Georgia with 16 electoral college votes and Utah on six are usually Republican and will all be key in a tight race.
Wisconsin on 10 votes and Michigan on 16 are Democrat heartlands and essential to a Clinton victory.
At 11 pm Irish time the first polls will close in Indiana, home to Trump's running mate and state governor Mike Pence. Along with Kentucky, with eight electoral college votes, both are Republic and a Trump win is expected.
At midnight the votes close in key battlegrounds Florida (29 votes), Virginia (13 votes), Georgia (16 votes), South Carolina (nine votes) and Vermont (three votes).
At half past midnight polls close in Ohio with 18 votes, North Carolina with 15 and West Virginia with five.
Trump has campaigned heavily in rust belt state Ohio, promising protectionist trade policies that appeal in a state with a declining manufacturing base.
At one am Pennsylvania polls close (20 votes). Likewise Alabama with nine votes, Connecticut with seven votes, Delaware with three, Washington DC with three, Illinois with 20, Michigan with 16. Kansas with six, Maine with four, Maryland with ten, Massachusetts with eleven, Mississippi with six, Missouri with ten, New Jersey with 14, Oklahoma with seven, Rhode Island with four and Tennessee with 11.
At two am Irish time, polls close in Colorado with nine votes, Wisconsin with ten,Texas with 38, Louisiana with eight, Minnesota with ten, Nebraska with five, New Mexico with five, New York with 29, South Dakota with three and Wyoming with ten.
Clinton should romp home in both New York and New Jersey where she is strongly favoured.
At 3am Irish time, voting closes in key battleground Arizona with eleven votes, Idaho with four, Montana with three, Nevada with six, Utah with six and Iowa with six.
At 4am, true blue state California closes with fifty five electoral votes to be decided, as does Washington State with twelve, Oregon with seven and North Dakota with three.
At 5am Irish time, polls close in Alaska with three votes and Hawaii with four.
With voters taking a dim view of both candidates, turnout will be key to this election.
Trump needs white working class voters to go to the polls in large numbers.
Clinton meanwhile is depending on young, female, black and Latino voters getting behind her in large numbers.
A large Latino turnout could be detrimental to Trump given his vocal anti-immigrant stance.