US election: How the electoral college system works
The BBC's Jeremy Vine explains how the US presidential election system works.
Tens of millions of Americans are voting to decide whether to re-elect Democratic President Barack Obama or hand the job to Republican Mitt Romney.
Each state is given a number of electoral votes in rough proportion to its population.
The candidate who wins 270 electoral votes - by prevailing in the mostly winner-take-all state contests - becomes president.
For in depth coverage as the US goes to the polls: BBC US election special report