US President Joe Biden and his predecessor Donald Trump meet tonight for the first of two televised debates that could prove critical in their tight race to win the White House in November.
Here is what you need to know:
When and where are the debates?
The first debate will air at 9pm US Eastern Time (2am Irish time tomorrow) on CNN and be broadcast from an Atlanta studio without a live audience.
It can also be viewed on CNN affiliates, without a cable login on CNN.com, and is available for simulcast on other US channels.
More than four months ahead of the 5 November vote, the event will be the earliest presidential debate in modern US history.
Television news channel ABC said it will host the second presidential debate on 10 September 10, also from an audience-free studio.
Who will moderate the debates?
Anchors Jake Tapper and Dana Bash will moderate CNN's debate.
ABC's will be moderated by anchors David Muir and Linsey Davis.
Who will debate?
CNN said that only Mr Biden and Mr Trump met its conditions for participating in the debate: appear on enough state ballots to potentially win the presidency and receive at least 15% in four separate national polls of registered or likely voters.
Independent presidential candidate Robert F Kennedy Junior did not make the cut.
ABC will also require debaters to appear on enough state ballots to reach the 270 electoral votes and receive at least 15% in four separate national polls.
What are the rules?
In tonight's 90-minute debate, candidates will appear at a uniform podium, be given a pen, paper and bottle of water and cannot use props or notes, CNN said.
"Microphones will be muted throughout the debate except for the candidate whose turn it is to speak" and moderators "will use all tools at their disposal to enforce timing and ensure a civilized discussion," it added.
President Biden will be on the podium on the right side of viewers' screens, and Mr Trump will get the last word after a coin toss.
Campaign staff may not interact with candidates during the two commercial breaks, and there will be no studio audience.
ABC's debate rules have not yet been announced.
What to watch for
US presidential debates often draw tens of millions of viewers, and through history have determined the course of some races.
This time, strategists say there are risks for both candidates, who are locked in a tight race and share low enthusiasm from voters.
Mr Biden, 81, and Mr Trump, 78, are the oldest candidates to seek the presidency and viewers are sure to question their cognitive abilities and emotional steadiness.
Mr Trump aides see Mr Biden as prone to verbal slip-ups that could amplify voter concerns about the president's age.
Mr Biden aides think debates could hurt Mr Trump by exposing his volatility and sometimes changing positions on issues, such as abortion.
Mr Biden's campaign is hoping to talk about abortion and Mr Trump's about immigration, but CNN's moderators will decide the questions.
Topics may include: how the candidates would handle challenges posed by persistently high grocery prices, migrants on the US-Mexico border, and the ongoing Israel-Hamas and Russia-Ukraine wars.
Mr Trump's role in the 6 January 2021 attack on the US Capitol, efforts to overturn the 2020 election and his conviction on 34 counts for involvement in a hush money scheme could come up during the debate.
The legal woes of President Biden's son, Hunter, who was convicted of lying about his drug use to illegally buy a gun, may also be raised.
Who will be there?
Campaign staff will be on hand to talk to reporters afterward in the "spin room".
The Biden campaign declined to comment on whether the president might bring any guests to the debate, but California Governor Gavin Newsom is expected to be there.
The Trump campaign is hosting a watch party in Atlanta, with rumoured contenders to be Republican vice presidential running mate - North Dakota Governor Doug Burgum, US Senators Marco Rubio and JD Vance, and others listed as "special guests."
What happened last time?
Mr Trump - as president - and Mr Biden debated twice during the 2020 race, with their first exchange degenerating into a chaotic shouting match.
According to Nielsen Media Research, more than 73 million people tuned in for their first debate in 2020.
For the second debate, moderators said they would mute each candidate's microphone to allow the other to speak without interruption for the first two minutes of each debate segment.
But the candidates behaved more civilly and the mute button was not a major factor.
A third debate was cancelled after Mr Trump tested positive for Covid-19 and spent three days in hospital.
He declined to participate in a virtual event.
Is that it?
Traditionally, there are three presidential debates. However, no third Trump-Biden matchup has been announced yet.
Vice President Kamala Harris accepted a CBS News invitation for a vice presidential debate on either 23 July or 13 August, according to a campaign spokesperson.
