Jill Stein, 66
Dr Jill Stein is the left-wing firebrand in this election, hitting both major party candidates hard on social media.
She has called Mr Donald Trump's campaign "essentially an elaborate con - like his business" and has criticised Mrs Hillary Clinton for bringing "mass incarceration, job crushing Nafta, & corporate sponsored elections".
A graduate of Harvard Medical School, Dr Stein has led initiatives to promote healthy communities and strengthen local green economies.
She is also an advocate for democracy.
Apart from being elected twice to "Town Meeting" - the local legislative body of the town of Lexington, Massachusetts, where she lives - she has little experience in elected office.
The mother of two, who was born in Chicago, made an unsuccessful bid for the White House in 2012 as the Green Party's candidate.
Gary Johnson, 63
Former governor of New Mexico Gary Johnson, 63, is on a mission to "make America sane again".
He said in a CNN town hall meeting that his role was to call out both sides in the election and provide a less polarising alternative: "If either Trump or Clinton is elected, things will be more polarised than ever."
The Libertarian Party candidate is socially liberal, a former chief executive of a legal cannabis company and has indulged in edible pot products, but has vowed to remain substance-free if he were president.
Often referring to himself as an entrepreneur, the father of two started a door-to-door handyman business to pay for his college studies, and grew it into a construction company with more than 1,000 staff. As governor, he cut taxes and balanced the state's budget.
His official website says he is an avid skier, adventurer, ironman and bicyclist, and has scaled the highest peak on each of the seven continents, including Mt Everest.