POLLS open at different times across the US today and will remain open for most of the day.
In many states people have been able to vote in advance with around one third of all votes already cast.
The main US TV networks will "call" each state for either Democrat Hillary Clinton or Republican Donald Trump.
Indiana and Kentucky are likely to declare first with polls closing at 11pm UK time. Both of these are safe Republican states, so ought to be called for Mr Trump. Polls in the rest of the states will close over the following hours, ending at 4am UK time.
The votes to watch are the so-called "swing states", which tend to vote different ways in each election.
Neither Mrs Clinton nor Mr Trump can win without taking at least some of these states. So watch out for Florida and New Hampshire, which should declare around midnight UK time, North Carolina and Ohio 30 minutes later, Arizona and Colorado at 2am and Iowa and Nevada at 3am.
Polls suggest Mrs Clinton is ahead in most of the swing states but not by much. If she holds all of the "safe" Democratic states, a win in Florida could be enough to see her over the finishing line: 270 electoral votes. In contrast, Mr Trump needs to take almost all of the swing states to get him past the winning post.
Each state has a number of votes based on population so while the very populous California has 55, the sparsely populated Montana has just three.
Normally, the result, if clear, is announced around 4am by the loser conceding. But recounts and legal challenges could hold things up for weeks. January 6 is the cut-off point when Congress validates the result.
The new, 45th, president, will be sworn in at noon on January 20.
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