Here is a guide to how the result of the US presidential election will be decided:
Q. What will happen on election day, November 8?
A. Polls open at different times across the United States and will remain open for most of the day. In many states people have been able to vote in advance and it is estimated at least one-third of all votes will have been cast before November 8.
Q. What happens when the polls close?
A. The main US TV networks will "call" each state for either Hillary Clinton (Democrat) or Donald Trump (Republican). Most states vote the same way each election and these will be called as soon as their polls close. Examples are New York and California for the Democrats, and Louisiana and Tennessee for the Republicans.
Q. Which states will be called first?
A. Indiana and Kentucky (polls close 11pm UK time). Both of these are safe Republican states, so these ought to be called for Mr Trump straight away. Polls in the rest of the states will close over the next few hours, ending at 4am.
Q. Which states are worth watching for?
A. 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. Here is when these polls are due to close (UK time): Florida and New Hampshire 12am; North Carolina and Ohio 12.30am; Arizona and Colorado 2am; Iowa and Nevada: 3am.
Q. Who currently has the advantage in these states?
A. Polls suggest Mrs Clinton is ahead in most of them, but not by much. She also has a slightly easier route to victory than Mr Trump. If she holds all of the "safe" Democratic states, a win in Florida could be enough to see her over the finishing line. Mr Trump needs to take almost all of the swing states to get him past the winning post.
Q. What is the winning post?
A. 270 electoral votes.
Q. And what are electoral votes?
A. Each state is worth a different number of electoral votes, based on population size - so California, a very populous state, is worth 55 votes while the sparsely inhabited Montana is worth just three. A candidate collects all the electoral votes for a state by coming top of the popular vote. For example, in Florida the polls suggest Mrs Clinton currently leads Mr Trump by around 47% to 45%. If this were repeated on election day, Mrs Clinton would collect all of Florida's 29 electoral college votes.
Q. When will we know who is the next president?
A. If Mrs Clinton or Mr Trump has won comfortably, it should be obvious by around 4am on Wednesday morning. If the result is close, the final picture might not be clear until late morning. But should some of the swing states be extremely close, recounts and legal challenges could hold things up for days or - as was the case with Florida in 2000 - weeks.
Q. How is the winner declared?
A. Usually by the loser conceding defeat. The actual process of counting votes takes many hours, sometimes days. If the overall result is close, do not expect anybody to concede any time soon.
Q. Is there a cut-off point when the result has to become official?
A. Yes: January 6 2017. This is when the US Congress meets to count and authorise the electoral votes.
Q. And Barack Obama stays in office throughout this period?
A. Yes, until 12pm on January 20 2017 when the president-elect will take the oath of office and become the 45th President of the United States.