The Republican and the Democrat have a tie in the Electoral College 269/269.
All the electors remain faithful and cast their electoral vote as their state voted.
The election of course goes to the House.
The House votes 25 state delegations for the Republican and 25 state delegations for the Democrat.
How is the tie in the House broken or what is the next step?
First of all, many states will split. When they split, they will be marked as "undecided", so the house does not need to break evenly along state lines. As Mr. Morden points out, the running mate selected Vice President by the Senate will become Acting President until such a time as the House appoints one of the candidates President. As jfern said, that would likely involve some kind of shady deal; think 1876.
