Some states historically support the Democratic candidate election after election, while others are just as resolutely Republican.
About a dozen states are prone to shift back and forth between the parties, often determining the final result, and the US election race will be won or lost in these swing states.
Polls suggest that Donald Trump has moved up in a number of these, but who can reach the 270 electoral college votes to win?
Florida is the biggest swing state of all, with 29 electoral college votes.
Trump must win in Florida if he is to have any chance of getting to 270.
If he doesn't win in Florida, this could all be over very quickly.
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North Carolina, with its 15 electoral votes, is a second must win state from Trump in addition to Florida.
Clinton can lose both Florida and North Carolina and still be elected, but Trump cannot lose either.
Pennsylvania, which has 20 electoral votes, is another one to watch and one which Clinton will not want to lose.
If Clinton were to lose Florida, North Carolina and Pennsylvania, she would be under pressure and the race would be wide open.
This scenario would also make small states such as Maine very important.
Maine allocates its four votes by electoral district, not winner takes all, and so both candidates will be looking for a few votes here, which could prove crucial in a tight race.
No Republican president has ever won the presidency without winning Ohio.
So Trump must take Ohio, along with North Carolina and Florida.
With polls showing him performing well in rural parts of the state, the irony is he could win Ohio but lose the election.
Nevada and Wisconsin could also become important, depending on what happens along the east coast.
East coast swing states are the ones to watch on Tuesday night.
