The United States Senate trial of President Donald Trump is well under way - and names like Volodymyr Zelenskyy, Joe Biden, Lev Parnas, among others are expected to repeatedly come up.

Trump was impeached on December 18 by the Democratic-led House of Representatives for abuse of power related to his dealings with Ukraine and obstruction of Congress for refusing to participate.

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Trump is only the third US president to be impeached.

Although Trump is expected to ultimately be acquitted in the Republican-controlled Senate, the trial is expected to last at least two weeks, and Democrats are pushing for witnesses.

As the trial continues, these are the key players in the coming impeachment saga:

The overseer

John Roberts

Supreme Chief Justice John Roberts, who turns 65 on January 27, will preside over the trial and arbitrate any disagreements among the senators about the process - although any decisions he makes can be overruled by a vote of the Senate.

Chief Justice of the United States John Roberts [Reuters]

A conservative appointed by George W Bush in 2005, it's Roberts's job to keep the trial on schedule by reining in any long-winded speakers.

He's made no public comments about the impeachment prior to the trial's start, but publicly rebuked Trump in 2018 when the president criticised an unfavourable ruling as coming from "an Obama judge".

The Jury

All 100 senators make up the jury. Republicans currently hold a 53-47 majority.

Mitch McConnell

As Senate majority leader, Mitch McConnell is responsible for setting the agenda for the impeachment trial.

Senate Majority Leader Mitch McConnell [The Associated Press]

McConnell, who has called Trump's impeachment "toxic", has admitted that he is "not an impartial juror" and wants a swift trial with no witnesses.

Chuck Schumer

Senate Minority Leader Chuck Schumer, the top Democrat in the Senate, wants a full trial with testimony from at least four current and former White House officials.

Democratic leader Senator Chuck Schumer [File: Matt Rourke/The Associated Press]

The candidates

Senators Elizabeth Warren, Bernie Sanders, Amy Klobuchar, and Michael Bennet, all of whom are running for the Democratic presidential nomination, would likely prefer to be out campaigning rather than tied to their Senate seats for an impeachment trial, but they have no choice.

US Democratic presidential candidate Senator Amy Klobuchar [File: Joshua Roberts/Reuters]

Senate rules require them to remain silent through much of the process, but expect them to be jockeying for television facetime at the end of each day.

The prosecution

Seven House managers will make the case against Trump. They include:

Adam Schiff, House Intelligence Committee chairman (team lead) Jerrod Nadler, House Judiciary Committee chairman Zoe Lofgren Hakeem Jeffries Val Demmings Jason Crow Sylvia Garcia

The Defence

Trump's defence team is made up of high-profile lawyers and many who have been loyal to him for years. They include:

Pat Cipollone (team lead) Jay Sekulow Pat Philbin Mike Purpura Alan Dershowitz Jane Raskin Ken Starr Robert Ray Pam Bondi Eric Herschmann

Potential witnesses

John Bolton

John Bolton served as Trump's national security adviser until he was forced out last year.

A lawyer for Bolton teased that his client has knowledge of "many relevant meetings and conversations" in the inquiry.

Former National security adviser John Bolton [File: Pablo Martinez Monsivais/The Associated Press]

Bolton has indicated that he would testify if subpoenaed, and Schumer has named him as one of four witnesses the Democrats would like to hear from.

Mick Mulvaney

Mick Mulvaney is Trump's acting chief of staff. Last year, Mulvaney admitted there was a quid pro quo (Latin for "this for that") in the administration's dealings with Ukraine.

Acting chief of staff Mick Mulvaney [File: Patrick Semansky/The Associated Press]

Mulvaney stunned reporters when he admitted that Trump partially froze the military assistance for Ukraine in part to pressure the Eastern European country to investigate Democrats.

Mulvaney later walked back on those comments, saying he did not make the admission.

Michael Duffey

The associate director for national security at the Office of Management and Budget, Michael Duffey held up the Ukraine aid despite being challenged on the legality of doing so by career officials in the office.

Robert Blair

A senior adviser for national security issues to Mulvaney, Robert Blair was on the now-infamous July phone call between Trump and Ukrainian President Volodymyr Zelenskyy.

Other players

Joe and Hunter Biden

Some Republicans have floated the idea of calling former Vice President Joe Biden and his son, Hunter, who served on the board of a Ukrainian gas company as witnesses.

There has been no evidence of wrongdoing by the Bidens, however.

Candidate Joe Biden [File: Paul J Richards/AFP]

Rudy Giuliani

Rudy Giuliani, Trump's personal lawyer, has been at the centre of the impeachment inquiry since its beginning.

Trump pressed Ukraine's new president, Volodymyr Zelenskyy, in a July 25 phone call to work with his attorney general, William Barr, and his personal lawyer, Rudy Giuliani, to investigate the Bidens and also a debunked theory that Ukraine, not Russia, interfered in the 2016 presidential election.

Former New York City Mayor and Trump's personal lawyer Rudy Giuliani [File: Shannon Stapleton/Reuters]

The Democrats' case rests in large part on a rough transcript of the July 25 call, in which Trump asks Zelenskyy to "do us a favour" and work with Barr and Giuliani in carrying out the investigations he sought. Current and former US officials testified that Trump directed them to work with Giuliani on Ukraine issues, despite the fact that the former New York mayor was a private citizen.

Nancy Pelosi

House Speaker Nancy Pelosi kicked off the impeachment inquiry in September. She handed off the case to the Senate earlier in January, but Trump continues to still rail against her on Twitter.

Speaker of the House Nancy Pelosi [Jim Lo Scalzo/Reuters]

Fiona Hill

Fiona Hill was a star witness during the House impeachment inquiry. She is the former senior director for European and Russian Affairs on Trump's National Security Council.

In her testimony, Hill recalled a conversation she had with Bolton about Giuliani's activities.

Former White House national security aide Fiona Hill [File: Andrew Harnik/The Associated Press]

Bolton, she said, looked pained and "basically indicated with body language that there was nothing much that we could do about it. And then, in the course of our discussion said Rudy Giuliani was a hand grenade that was going to blow everyone up".

Asked what she thought Bolton meant, Hill replied that Giuliani was "pretty explosive" and that the former New York mayor "was clearly pushing forward issues and ideas that would, you know, probably come back to haunt us and, in fact, I think that that's where we are today."

Gordon Sondland

The US ambassador to the European Union, Gordon Sondland, provided some of the most damaging testimony before the House Intelligence Committee. He said he spoke directly with Trump about the effort to pressure Ukraine and said other top administration officials were involved. He testified that Ukrainian officials understood they would have to announce the investigations in order to get the withheld security aid.

US Ambassador to the European Union Gordon Sondland [File: Susan Walsh/The Associated Press]

William Taylor

William Taylor, the former top American diplomat in Ukraine, described in his testimony when he learned that Trump's officials or agents conditioned nearly $400 million in US aid to Ukraine and a White House meeting on Ukraine conducting political investigations.

"By mid-July, it was becoming clear to me that the meeting [Ukrainian] President [Volodymyr] Zelenskiy wanted was conditioned on the investigations of Burisma and alleged Ukrainian interference in the 2016 US elections," Taylor testified last year, referring to the Ukrainian gas company where Hunter Biden served on the board.

"I think it's crazy to withhold security assistance for help with a political campaign," Taylor recalled telling US officials.

Top US diplomat in Ukraine William Taylor [File: Andrew Harnik/The Associated Press]

Taylor stopped short of directly implicating the president, saying he believed the push for political investigations was coming from Rudy Giuliani, Trump's personal lawyer, but had not spoken to Giuliani or Trump about it.

"I think it was coming from Giuliani," he said, referring to Trump's lawyer. "I don't know what was in the president's mind." Trump has said he did nothing wrong.

Marie Yovanovitch

Marie Yovanovitch, a widely respected career diplomat and former US ambassador to Ukraine, had spoken out in Kyiv against the oligarchy still controlling Ukraine's economy.

Yovanovitch was recalled in May after being informed by Deputy Secretary of State John Sullivan that she had "done nothing wrong", but that Trump had "lost confidence" in her, according to a transcript of her testimony.

Marie Yovanovitch, former US ambassador to Ukraine [File: Andrew Harrer/Reuters]

Trump spoke negatively of Yovanovitch in his phone call with Zelenskyy.

"The former ambassador for the United States, the woman, was bad news and the people she was dealing with in the Ukraine were bad news, so I just want to let you know that," Trump told Zelesnkyy, according to White House notes of the call.

Yovanovitch told House investigators on October 11 that she felt threatened by Trump describing her on the call to Zelenskyy as "bad news" a transcript showed.

"I was very concerned," she said. "I still am."

Other individuals who testified in the House impeachment inquiry: