Three hundred million Americans, 50 states, two candidates and well over $2bn in campaign spending. How does this election work?
The campaign comes down to one number: 270. That's the number of electoral college votes both sides need to win to claim the White House. The electoral college was created in the earliest days of the US as a voting system that allows the disparate states to come together and elect a single president to represent them all.
Each state, plus Washington DC, is awarded a certain number of electoral votes based roughly on size. All but two states use a winner-takes-all system, so if you win the most votes in a state you take its entire haul of electoral college votes. There are 538 electoral college votes in total and the candidate to get a majority -- 270 -- becomes president.
So how do they go about reaching 270?
By targeting "swing states" -- a small group of around a dozen states that sometimes vote Democrat and sometimes vote Republican.
The vast majority of America's states are considered solidly committed to one party, but others, like Florida, Ohio and Iowa, are open to persuasion. The campaigns will focussed their time, their money and their resources on winning these swing states.
What happens on Election Day itself?
Yesterday, polling booths opened in all 50 states and in Washington DC. Counting begins immediately and when voting finishes in the evening we will get our first glimpse of the exit polls.
Usually by around 11pm on the East Coast (4am GMT) it has become clear that one side has prevailed. In that case the losing candidate calls the winner to concede. Both men will give a speech: one to claim victory and the other to admit defeat.
But there is always the possibility -- as happened in 2000 -- that at the end of Election Day we may still not know who's won. The result could either be too close to call without counting every vote or else legal battles over election procedures may delay the result or force a recount.
It could even be a tie, with both candidates stuck at 269, in which case the House of Representatives would vote to choose the next president.
If Mitt Romney wins does he become president straight away?
No. The next president will be publicly sworn into office on January 21, 2013.
If Mr Romney wins he'll spend the time between November and January -- known as "the transition" -- assembling his cabinet, his White House staff and preparing for government. If Mr Obama wins his administration will largely stay in place, although he may decide to reshuffle his cabinet and aides for his second term.
Are voters just casting ballots for the presidential election?
No. As well as voting for president, Americans are also electing all 435 members of Congress's lower house, the House of Representatives, and one-third of the Senate. Plus, they are voting for a medley of local and state officials.
There has been a lot of talk about "SuperPACs". What are those?
SuperPACs (super political action committees) are a new phenomenon that are having an extraordinary influence on this election. They are independent political groups -- in theory not connected to any candidate -- that are allowed to raise and spend unlimited amounts of money during the campaign.
By law, SuperPACs must be completely separate from the campaigns. But the reality is that the SuperPACs act as aggressive proxies for the official campaigns, echoing their message and hammering their opponents.
The concept of the SuperPAC emerged from a landmark Supreme Court case, Citizens United v Federal Election Commission, where justices ruled that there could be no limits to how much money individuals, corporations or unions can donate to independent political groups, even if they are campaigning on behalf of one candidate. (© Daily Telegraph, London)