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One of the trial's key players for the Democrats is Congressman Jamie Raskin, who is lead impeachment manager.

It's a huge job under any circumstances, but the 58-year-old former law professor has had an extremely difficult year so far.

On New Years' Eve his son Tommy Raskin, 25, took his own life and was buried in early January.

"Tommy Raskin had a perfect heart, a perfect soul, a riotously outrageous and relentless sense of humor, and a dazzling radiant mind," Raskin and his wife wrote in a tribute on 4 January., external

Then on 6 January, Raskin found himself caught up in the violent attack on Congress and was forced to hide from rioters in the Capitol building.

Later that day, he helped to draw up an article of impeachment against Trump.

He has said that his son, external, who was studying law at Harvard University, would have considered last month's violence "the absolute worst form of crime against democracy".

"It really is Tommy Raskin, and his love and his values and his passion, that have kept me going," he explained.

His job this week will be to guide the prosecution of Trump through the Senate.

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Donald Trump is not only facing allegations of wrongdoing within the impeachment trial.

The office of Georgia’s secretary of state says it has opened an investigation into the former president's efforts to overturn the state's 2020 election results.

The investigation focuses on a phone call in which Trump was recorded telling the state’s top election official to "find" enough votes to overturn the result.

"I just want to find 11,780 votes,” Trump told Republican Secretary of State Brad Raffensperger in the recording released last month by the Washington Post, external.

Raffensperger is heard replying that Georgia's results - in which Democrat Joe Biden was victorious - are correct.

In a statement to US media, Raffensperger’s spokesman said the secretary of state’s office “investigates complaints it receives”.

“The investigations are fact-finding and administrative in nature. Any further legal efforts will be left to the Attorney General.”

Jason Miller, a senior adviser to Trump, has said there was “nothing improper or untoward about a scheduled call between President Trump, Secretary Raffensperger and lawyers on both sides", according to AP news agency.

You can read more about the phone call here.

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Security forces around Washington DC are on high alert ahead of Donald Trump's second impeachment trial.

Members of the National Guard have remained on patrol around the outside of the US Capitol since the 6 January siege - the deadly riots that led to Trump's trial today.

And all of the Democratic impeachment managers will be provided with a security detail for the duration of the trial (this happened at the first impeachment, too).

Law enforcement has said they are not following any "specific and credible" threats related to impeachment, but two weeks ago, the US Department of Homeland Security warned of a "heightened threat environment across the US", expected to continue in the weeks following Joe Biden's 20 January inauguration.

Reality Check

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Donald Trump’s lawyers have filed a 78-page brief setting out his defence, external.

They claim the whole process is unconstitutional as Trump is no longer president.

It’s true that the US Constitution doesn’t specifically deal with this.

However, the prevailing legal view, external is that if an impeachment process begins when an individual is in office (as Trump was), then it can continue after they depart.

His lawyers also deny he incited violence, claiming the president "did not direct anyone to commit unlawful actions".

They point out that during his speech on 6 January just before the storming of the Capitol, the president told protesters to “peacefully and patriotically make your voices heard", but he also used the word “fight” in that speech 14 times.

Although Trump never explicitly directed his supporters to enter the Capitol building, the Democrats say there is clear evidence that “President Trump was comfortable urging, approving, and even celebrating violence".

His lawyers say the rhetoric was in regards to fighting for more election security, and claim he had the right to express his opinion on the election result and other matters.

Read more.

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Just two hours ahead of the trial, Senate Majority Leader Chuck Schumer is asking Republican Senators to listen carefully to the case against Donald Trump.

"I urge all of my colleagues to pay careful attention to the evidence. I particularly urge my Republican colleagues - despite the pressure on them - to pay very real attention to the evidence here because it is very, very serious."

The New York Democrat also said he expects the House impeachment managers to "present a very strong case" with some new compelling evidence - though he did not allude to what that evidence might be.

Though the real fireworks don't start until Wednesday, today is a big day as it opens Trump's unprecedented trial in the Senate.

• At 1300 EST (1800GMT) the proceedings will start, chaired by Democrat Patrick Leahy
• Debate will last four hours, equally divided between both sides. They will be arguing about whether the trial is constitutional
• A vote by simple majority will follow, which is expected to pass

Chelsea Bailey
Digital producer, BBC News

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Seems to be strange that they say they can't impeach a president that has left the office. Does this mean that any American president can do anything during their last days in office and get away with it? - Kevin, Oxford, UK

Kevin, in many ways, this impeachment trial is about getting to the heart of your question: What precedent is the US setting as a country if these actions stand? For better or worse, Donald Trump has been a trailblazer of a politician. And while he’s often given a pass for sidestepping nearly all political norms, the Capitol riots seem to have been a step too far for many Americans as well as politicians.

But whether or not the US Constitution allows a former president to be impeached remains to be seen. Lawyers for Donald Trump will make the case that it can't be done. Democrats, unsurprisingly, reject that argument.

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To convict Donald Trump, Democrats will need to muster a two-thirds majority in the Senate. With the chamber split exactly in half - 50 seats each - at least 17 Republicans will have to vote with Democrats to make this happen.

This isn't likely.

Still, there are some Republicans to keep an eye on, who might cross party lines in the trial.

1. Mitt Romney, from Utah. Voted to convict Trump last time, has said the riot was "an insurrection incited by [Trump]"

2. Ben Sasse, from Nebraska. Has been an outspoken critic of Trump, saying the 6 January violence "was the inevitable and ugly outcome of the President's addiction to constantly stoking division".

3. Susan Collins, from Maine. A moderate Republican, she wrote an op-ed saying Trump had "incited" the rioters.

4. Lisa Murkowski, from Alaska. She called on Trump to resign as president following the riots.

5. Pat Toomey, from Pennsylvania. He has said that Trump might face "criminal liability" for the 6 January siege.

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Donald Trump is facing an impeachment trial for the second time, but this time he'll watch the drama play out from West Palm Beach, Florida, instead of the White House.

The former president has kept a low profile since leaving office last month, spending time at his Florida estate Mar-a-Lago. And his indefinite ban from Twitter has meant we've heard far less from the Republican than we got used to during his four years in office.

But unfortunately for Trump, he didn't leave all of his legal disputes behind in Washington. Officials in Palm Beach will meet tonight to discuss if he's allowed to keep living at his Mar-a-Lago resort. Some of his neighbours have argued he's violating a decades-old agreement by residing at the private club.

Lawyers for both sides will have to make constitutional arguments to win their case.

But lawyers for the Democrats - looking to convict Republican Donald Trump - are hoping to avoid dry legal analysis. Instead, they'll focus on video footage to evoke the trauma and emotion of that day when Trump supporters stormed the US Capitol.

The footage of 6 January will be sure to include both the violence and the speech from the former president that preceded it.

This way, the Democrats will be speaking to both the Senators in front of them, and the Americans watching from home.

Watch here as three members of the US Congress - from both parties - describe in vivid detail what it was like being there.

The proceedings beginning today have a range of possible outcomes - all the way from Trump being found not guilty to him being barred from running for president again.

Our colleagues in Visual Journalism have put together a handy graphic explaining the process.

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Being the most powerful official in the land, you might expect the president to be playing a role in this trial.

But Joe Biden has been keeping a low profile, and as Congress is a separate political body to the White House, the president is not involved in the proceedings. He does not even plan to watch much of it, his press secretary Jen Psaki said on Monday.

"The president himself would tell you that we keep him pretty busy and he has a full schedule this week,” Psaki told journalists.

Overall Biden has said very little about the impeachment trial and said on Monday that he would allow the Senate to determine Trump's fate.

When will the final vote be? It's thought that due to a speedy timetable put forward by politicians, the Senate could vote whether to convict as early as Monday.

But before then each side will put forward their arguments in a schedule that looks like this:

Tuesday: Four hours for both sides to present arguments about whether the trial is constitutional, followed by a vote (which is expected to pass). In what's expected to be one of their core arguments, Trump's lawyers have said it is unconstitutional because Trump has left office and is now a private citizen.

Wednesday: Arguments for the trial begin, with each side being given 16 hours over the following days.

Weekend: Opening arguments are expected to be over, and Senators will be able to post questions to both sides.

Politicians on both sides are said to want a speedy trial.

You might recall that Trump's first impeachment Senate trial, in early 2020, took much longer - around three weeks.

It's worth remembering the maths in all of this. A two-thirds of the 100-seat Senate is required to convict Trump - Democrats hold 50 seats and there is little sign enough Republicans will join them.

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Donald Trump is the only US president to be impeached twice and only one of three to ever be impeached at all. Who were the others?

Andrew Johnson was the first in 1868. He was accused of, among other things, dismissing his secretary of war against the will of Congress. Mr Johnson had a narrow escape - the two-thirds majority in the Senate was missed by just one vote.

Bill Clinton found himself impeached on the grounds of perjury and obstruction of justice after he lied about the nature of his affair with Monica Lewinsky and then allegedly asked her to lie about it as well. When the trial reached the Senate in 1999 the vote for a conviction failed to get close to the two-thirds backing required.

And Richard Nixon, the 37th president, resigned in 1974 before he could be impeached over the Watergate scandal.

Find out more about what happens to impeached presidents.

Jon Sopel
BBC North America Editor

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The Trump defence will take two forms.

On the substance of the "incitement of insurrection" charge, his lawyers will argue that he was exercising his free speech, First Amendment rights - and they will point out that in that address on 6 January, the president told his supporters to march on Congress "peacefully and patriotically".

But the speech was notable for all its "we fight like hell and if you don't fight like hell, you're not going to have a country anymore". And telling his supporters that they have to be tough and not weak.

And his case is not helped by the tweets and messaging around this time - urging his supporters to come to Washington on 6 January because it was "going to be wild".

So don't expect the president's words to be the backbone of the defence.

Instead it will focus on the constitutionality of impeaching a president once he's left office. The lawyers will argue that the weapon of impeachment is only to be used for a serving politician, not a private citizen (as Donald Trump now is).

How can you use the sanction of removing someone from office when they've already left office? And this is I suspect the justification (fig-leaf, I feel sure Democrats will insist) that Republicans will reach for as their justification for acquitting Donald Trump.

Read more of Jon's analysis.

Anthony Zurcher
BBC North America reporter

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It would take 17 Republicans joining all the Democrats to successfully convict Donald Trump in the Senate. All but a handful of the Republicans seem to view the entire trial as unconstitutional, so the Democrats’ best strategy may be to make an acquittal vote, even if it is a fait accompli, as uncomfortable as possible.

They'll do their best to remind the 100 senators of the fear, confusion and anger they felt on 6 January, as they fled the chamber in the face of an angry mob.

They'll try to lay responsibility for the riot at the feet of a president who spent two months questioning the outcome of the election and chose to hold a protest rally within walking distance of the Capitol on the day Congress gathered to certify that election.

And then they'll have to hope that history - and American voters - are willing to assign guilt, even if the Senate isn't.

Image source, Getty Images

Just to recap on the events of 6 January, five people died in the rampage including a police officer.

Investigators have since pored over pictures and footage of the events to bring those responsible to justice.

As of last Friday, about 190 people had been charged with federal crimes, the Associated Press reported.

In addition, the FBI has opened more than 400 other files to try to identify other suspects.

Some of those arrested face charges of conspiracy which carry up to 20 years in prison if convicted. They include members of the Oath Keepers paramilitary group and the far-right Proud Boys group.

Donald Trump's accusers say you have to go back to November's presidential election and his unproven claims that the vote had been rigged. He lost but refused to admit it.

Fast forward to 6 January and thousands of Trump supporters gathered in Washington DC for a "Save America" rally organised to challenge the election result.

In a long address near the White House, Mr Trump reiterated his claim that he won the election "by a landslide" and encouraged his followers to "stop the steal".

"We fight like hell and if you don’t fight like hell you're not going to have a country any more," he said. However, his defenders point out that he also told those who were about to march on the Capitol to "peacefully and patriotically make your voices heard".

Read more about what he said here.

Image source, Reuters

The Senate's 48 Democrats, two independents, and 50 Republicans will be the ones casting the votes about whether to convict Trump.

The chamber's longest-serving member Democrat Patrick Leahy, 80, will be chairing.

And making the case for the prosecution will be the lead impeachment manager Democratic Representative Jamie Raskin. The 58-year-old former law professor has had a difficult start to the year - his son took his own life on New Year's Eve and then the Congressman was caught up in the riots on 6 January.

Also expected to play a key role is Mitch McConnell, the Senate's leading Republican. He initially castigated Mr Trump for the attack on the Capitol, but later voted to try to stop the trial going ahead.

Read more about who's who in the trial.

Image source, Reuters

The former president is accused of “incitement of insurrection against the Republic he swore to protect” - namely the storming of Congress by a mob of his supporters on 6 January.

Democrats in the House of Representatives say his repeated refusal to concede the election to Joe Biden encouraged the riot.

They say he should be disqualified from ever running for office again. If the Senate does impeach Trump, that opens the door to a vote that could ban him from running again.

But Trump's lawyers say that because he is now a private citizen the trial is unconstitutional.

They also say the proceedings are in violation of his right to free speech.

On Monday, they argued that FBI documents show the riot was planned days in advance, meaning Trump cannot be held responsible.