Barack Obama has cast his ballot at a school in Chicago, the city where his political career began.
Accompanied by his wife, Michelle, and their two daughters, the Democrat front-runner voted electronically.
Obama worked here as a community worker and then lawyer, before being elected as a state senator in 1996.
Obama went on to Indianapolis for some last-minute campaigning, and will then watch the results come in at a secret location.
The Democratic vice-presidential candidate, Joe Biden, voted near his Delaware home with his wife and elderly mother, before travelling to the battleground city of Richmond, Virginia.
After the final results are announced, Obama will address a huge crowd at Grant Park in Chicago. Officials estimate that up to a million people will attend.