The upper chamber of the US congress will see high-profile legal teams battle it out to prove their side is right, with the future of the president on the line.
Democrats in the US house of representatives impeached Donald Trump on two charges: first, the abuse of power by asking Ukraine to investigate Trump's presidential rival Joe Biden while refusing to withhold aid. The second, an obstruction of congress by refusing to cooperate with their investigation.
The senate opens with a debate on the structure and rules of the trial.
Senate majority leader Mitch McConnell has proposed a condensed, two-day calendar for opening arguments on the articles of impeachment.
The Republican's ground rules must be voted on as one of the first orders of business.
It pushes back any votes on witnesses until later in the process, rather than up front, as Democrats had demanded.
Will it be televised?
As per Senate rules, proceedings can be debated in private, meaning the television cameras and anyone who is not a member of the senate will be asked to leave the chamber.
This was done during Bill Clinton's impeachment trial.
Mr McConnell has argued that members of the chamber listen to each other better in private.
How long could the trial last?
After the four days of opening arguments – with a maximum of 24 hours per side – senators will be allowed up to 16 hours for questions to the prosecution and defence, followed by four hours of debate. Only then will there be votes on calling other witnesses.
Senate rules say the trial must proceed six days a week - all but Sunday - until a ruling is delivered.
Presidential hopefuls to have their say
Senators Elizabeth Warren of Massachusetts, Bernie Sanders of Vermont, Michael Bennet of Colorado and Amy Klobuchar of Minnesota are expected to be in the US upper chamber, but are also vying for the right to take on Trump in 2020's presidential election.
They'll be juggling their own presidential hopes by securing the Democratic nomination for president, while also holding the fate of the current president in their hands.
Who is prosecuting Trump?
Adam Schiff of California will be leading the case against Trump, along with judiciary committee chairman Jerrold Nadler of New York.
Five other Democrats round out the prosecution team, a group US house speaker Nancy Pelosi said she chose in part for their experience with the law.
Also on the prosecution team include: Hakeem Jeffries of New York, Sylvia Garcia of Texas, Val Demings of Florida, Jason Crow of Colorado and Zoe Lofgren of California.
Who is defending the president?
Donald Trump has picked a star-studded line up to form his defence team.
White House counsel Pat Cipollone and personal lawyer Jay Sekulow are expected to lead the argument that Mr Trump committed no crimes, that abuse of power is not an impeachable offence and that the president is a victim of a political “witch hunt” by Democrats.
Bringing experience both in constitutional law and the politics of impeachment, he has added retired law professor Alan Dershowitz and Ken Starr, the independent counsel who investigated Mr Clinton. The team also will include Pam Bondi, the former Florida attorney general.
The team, less experienced in the senate than the house prosecutors as a whole, visited the senate chamber on Monday, in part to test the equipment they expect to use for audio-visual presentations.
Look out for signs of tension involving the president’s outside legal team and lawyers within the White House. On Sunday, Mr Dershowitz tried to distance himself from the president.
100: The total number of senators.
53: The Republican majority.
51: The number of senators who must agree on almost anything to make it happen during an impeachment trial.
Four: The number of Republican senators who must join the Democrats to get to the magical 51 level.
Two thirds: The proportion of senators required to convict and remove a president from office.
So, 67 members of the Senate would have to vote to convict, if every senator is voting.
Senators to keep an eye on
Both sides will be keeping tabs on the senate’s moderates for an emerging gang of three to four who could influence the outcome on such matters as whether to subpoena former national security adviser John Bolton. That vote will not be taken for days, if not weeks.
Republican senator Susan Collins of Maine has been meeting with a small number of her party colleagues who want to consider witness testimony and documents that were not part of the house impeachment investigation.
Watch Republican senators Mitt Romney of Utah, Lamar Alexander of Tennessee and Lisa Murkowski of Alaska for signs of whether this group can stick together and force the senate to consider additional material.