They have been a set-piece of presidential election campaigns for more than half a century, producing many memorable and iconic moments.
But as President Donald Trump and his Democratic opponent Joe Biden prepare to face off tonight in their final TV debate ahead of the November 3 poll, what impact might it have on the American people?
The first televised debate was John F Kennedy v Richard Nixon on September 26, 1960.
And while what the candidates said has been largely forgotten, their clash is remembered for Nixon's visible sweaty appearance next to his more composed rival. Those who listened on the radio thought Nixon had won - but those watching on TV gave it to Kennedy.
Suddenly, image was everything.
Dr Mitchell S McKinney, a national and international expert on presidential debates, who has been studying them since the late 1980s, says that is what made the format disappear for a time.
He said: "The debates went away just a bit until we got to 1976. And that was primarily because of Richard Nixon feeling that it was his performance in the debates and viewers seeing him side by side with John Kennedy that really led to his defeat in that 1960 election."
He added: "And sometimes candidates have refused. Jimmy Carter refused until just nine days before the election with Ronald Reagan, who turned in a very strong debate performance and went on to win in a landslide."
Since they returned in 1976, they have been held in every presidential campaign.
Sharp exchanges became famous moments. In 1980 Reagan quipped "I am paying for this microphone" after it was turned off during a debate.
Four years later, against Democratic opponent Walter Mondale, when asked if at 73 he was too old to be President, he responded: "I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience."
Sometimes saying nothing at all created a stir in itself - when Al Gore was mocked for his continual sighing during his 2000 debate with George W Bush.
Perhaps the most memorable exchange of all came in the 1988 vice-presidential debate, when Republican Dan Quayle was humiliated after an ill-judged attempt to draw on the Kennedy legacy.
Comparing his life in politics to that of Kennedy, his Democrat opponent Lloyd Bentsen's cutting response was a "drop the mic" moment before its time. He responded: "I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy."
In the last election Trump and Hillary Clinton broke audience records - around 84m watched their first debate.
The current race for the White House between President Trump and Mr Biden saw one of the most chaotic head-to-heads, as each launched personal attacks and talked over the other.
It led to new rules being introduced to mute microphones in the final event tonight.
Dr McKinney said that the debates have different roles, and can have a big impact on the small portion of the electorate who are undecided. One of the key things for voters is that the debates show the candidates in a place where they are "not in control of the messages".
He added that it is also one of the only times where the major party candidates come together, side by side, during the long presidential campaign.
"Voters really value that opportunity to compare the candidates," he added.
"Under equal conditions, in a general election debate we expect to hear equally from the candidates in a debate, and voters tend to think of this as the debate moment as a useful, credible form of campaign communication, as opposed to the candidates' ads, as opposed to the candidates' controlled convention addresses, or their stump speeches, or perhaps their media appearances on partisan media or their chosen media network."
And it is this lack of control of the narrative that voters attribute to it being a "credible form of information".
He said: "Now, that credibility that voters point to in the debate message I think is driven largely by the fact that candidates are not in control of this message. Candidates show up without notes, without teleprompter, without their aids around them. They don't know what questions are going to be put to them. And therefore, in that moment, voters see how they respond to a journalist's question, how they respond to one another when they are attacked.
"I think that leads to this notion of getting the real, or the authentic, candidate, which although there is issue discussion, and there is issue learning from the debate, the greatest form of learning or the impression that debates leave is on candidates' character or image of how they react to one another, how they respond, how they deal with attacks. And that voters find to be a useful feature of debates."
They have always had a great reach, and this has remained consistent over the years.
Dr McKinney added: "Certainly, when we first started debates in terms of the size of the US population back in 1960, the debates had great appeal, great reach. About 80% of the US adult population viewed or listened to at least one of those Kennedy-Nixon debates." Interest dipped slightly in the mid-90s debates between Bill Clinton and Republican challenger Bob Dole, but interest has returned.
He said: "Eighty-four million in that first Clinton-Trump debate four years ago set the all-time record.
"That number of viewers actually gets us close to things like Superbowl viewership. This was a political event. Of course, many partisan voters' viewers may use the debates, like the Superbowl event, to tune in and to cheer on their chosen candidates."
Dr McKinney says that about 90-95% of viewers come to the debate with their minds made up - and typically are not changed.
But importantly, he says they matter for the very small slice of the "undecideds" or the "persuadable" in battleground states where elections are won and lost.
He said: "Debates have shown their ability to reach that very small slice of the undecideds, uncommitted, the persuadables.
"And of that group, consistently 3% to 4% will come out of viewing a debate, again 3% or 4% of 87m, claiming that they now have committed, they now know who they're going to vote for. And one thing we know about viewers is they are more likely to show up and vote if they're going to commit the time to watch a debate.
"So, when the race is close enough again, in battleground states, and there's enough undecided voters, then the debate I think, can be consequential, even consequential in the outcome of an election."