Really hoping this does not happen...

What Happens If the Presidential Election Ends in a Tie?

"Under Article 2 of the Constitution, the U.S. House of Representatives would be responsible for breaking the tie. When the newly-elected House is seated in January, lawmakers would take a vote."

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Won't happen according to Nate Silver:

"Statistician Nate Silver, who runs The New York Times blog Five Thirty Eight and correctly predicted the outcome of 49 out of 50 states in the 2008 race, has put the chance of a tie in Tuesday's presidential election at just 0.03 percent."

And a nifty video complete with helpful arrows and stick figures!