9 October 2016 Last updated at 10:18 BST
But one of them, Donald Trump, has landed himself in some big trouble after comments he made about women 11 years ago were released.
In the past Donald Trump has used words like "pig", "fat" and "ugly" to describe women he disagreed with.
Now a 2005 video has come out, in which he said even worse things about women.
Since the recording was made public, Donald Trump has released a video statement where he apologised for the comments.
But Hillary Clinton has said that Trump should not be president after what he has said.
She says that his words will have a negative effect on women and girls.
People in Donald Trump's own party have said they strongly disagree with the things he has said. Some of them have said they will no longer be supporting him in the election.
The president of the United States is one of the most powerful and important jobs in the world, and the winner of this year's election will take over from current president Barack Obama.
These big presidential debates are shown on TV to try to help people make their mind up about who they want to vote for in the election.

The government has announced contracts with Asian companies worth $4.5bn billion dollars.
President Ali Bongo signed the agreements on the eve of the country's 50th anniversary of its independence from France.
The projects will revamp infrastructure and create about 50,000 jobs.
Gabon's oil output has been declining for years and the new deals are presented as the first steps towards diversification.
The largest deal is the with OLAM, a Singapore-based company which plans to develop a huge palm oil plantation in the south east.
The government wants to become the leading palm oil producer in Africa.
In terms of infrastructure, the OLAM deal involves the construction of a refinery, and there are plans for a possible port.
Another agreement was signed with the Indian company M3M,  to build 5,000 low-cost housing units over the next two years.
Ramky Infrastructure, another Indian construction company, will build 1,000 kilometres of tarred road over the next three years in a $1.5bn project that will be jointly financed by the Gabonese government.
Gabon has already attracted a number of Asian investors, notably Malaysian and Chinese companies, but most of them were interested in the mining and timber sectors.

Artillery and small arms fire were used in the town of Laukkai, in the Kokang region.
Fighting reportedly erupted after rebels dressed in police uniforms launched a surprise raid.
Five civilians, five police officers and at least 20 rebel fighters are said to have died.
The office of Myanmar's de facto leader, Aung San Suu Kyi, said in a statement: "According to initial information, many innocent civilians including a primary school teacher... were killed because of attacks by the MNDAA armed group."
The Myanmar Nationalities Democratic Alliance Army (MNDAA) is a rebel group fighting Myanmar's armed forces in the region.
Officials said the attack was launched early on Monday, and targeted police and military posts. A separate group of fighters later attacked other sites in Laukkai.
Video shared on social media appeared to show parts of the town on fire on Monday afternoon, as civilians ran to safety.
An army source told the AFP news agency that the fighting was ongoing as darkness fell.
"Residents in town are fleeing. We do not know exact figures yet," the officer said.
What is Myanmar like?
The Northern Alliance, a multi-faction rebel group which includes the MNDAA, confirmed on Facebook that its members were fighting in Laukkai.
It said the attack was carried out in "self-defence", citing operations by the Myanmar military since December.
The violence is among the worst to hit the Kokang region, in the northern part of Shan state, since 2015.
The worsening conflict in the borderlands has raised tensions between Myanmar and much larger China.
Kokang has close ties to China, with locals speaking a Chinese dialect and using the yuan as currency.
Ms Suu Kyi's government is desperate to end the decades-long violence, amid fears it could spark a new refugee exodus.
A big debate takes place on Sunday night in America between Hillary Clinton and Donald Trump, the two people trying to become the next US president.
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Gabon has signed deals to diversify its economy in an attempt to be less reliant on its dwindling oil reserves.
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At least 30 people have been killed on Myanmar's border with China, after clashes between ethnic rebels and the security forces.