Autor:Americans are voting in one of the most divisive presidential elections in decades, pitting incumbent Republican Donald Trump against his Democratic challenger Joe Biden.
The first polls opened from 05:00 EST (10:00 GMT) in Vermont.
Nearly 100 million people have already cast their ballots in early voting, putting the country on course for its highest turnout in a century.
Both rivals spent the final hours of the race rallying in key swing states.
National polls give a firm lead to Mr Biden, but it is a closer race in the states that could decide the outcome.
Among the first states to begin election-day voting on Tuesday are the key battlegrounds of North Carolina and Ohio (11:30 GMT), followed half an hour later by Florida, Pennsylvania, Georgia, Michigan and Wisconsin. Arizona will follow at 13:00 GMT.
To be elected president, a candidate must win at least 270 votes in what is called the electoral college. Each US state gets a certain number of votes partly based on its population and there are a total of 538 up for grabs.
This system explains why it is possible for a candidate to win the most votes nationally - like Hillary Clinton did in 2016 - but still lose the election.
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