It's been more than 160 years since the US had a president who was not a Republican or a Democrat.
Many different parties field candidates at each election, and few attract any real support.
But this year's candidates, Donald Trump and Hillary Clinton, are among the most unpopular presidential nominees in US history.
That is why as many as 10 percent of voters say they will choose a third candidate in November.
Inside Story looks at the two most well-known: Libertarian candidate Gary Johnson and Jill Stein of the Green Party.
Presenter: Jane Dutton
Joe Watkins - Republican political strategist and former White House aide to President George W Bush
Jeanne Zaino - Professor in politics at Iona College, New York
Cliston Brown - Political analyst and columnist for the New York Observer.
Source: Al Jazeera