The Democratic Party is planning to hold an inquiry into whether President Donald Trump should be impeached over his contacts with Ukraine about the former vice-president, Joe Biden.
US President Donald Trump is facing a process that could eventually see him removed from office.
There were some calls for his impeachment during the years-long Russia inquiry, but this time House Speaker Nancy Pelosi has gone further and set up a group to look into it.
It all centres on whether or not he improperly sought help from Ukraine to boost his chances of re-election in 2020.
Legal scholars suggest presidents cannot be prosecuted while in office, so the only way he could be removed from the job would be by impeachment.
Things are still at an early stage. The first public hearings started on Wednesday in the lower house of Congress, the House of Representatives. That is controlled by the Democrats. President Trump, who is a Republican, strongly denies any wrongdoing.
But just how does that happen? And exactly who has been impeached in the past? The answer may surprise you...
Depending on what happens in the next few weeks, Mr Trump could end up facing impeachment - but more on what that means below.
What is he accused of doing wrong?
President Trump is accused of pressuring Ukraine's President Volodymyr Zelensky to dig up damaging information on one of his main Democrat challengers, Joe Biden, and his son Hunter.
Hunter worked for a Ukrainian company when Joe Biden was US vice-president.
Asking foreign entities for help in winning a US election is illegal.
What is the evidence?
At the heart of the story is a complaint from a whistleblower - an unnamed intelligence official - who wrote a letter expressing concern about Mr Trump's 25 July call with Mr Zelensky.
A rough transcript of the call revealed that Mr Trump had urged President Zelensky to investigate discredited allegations against Joe and Hunter Biden.
The call came shortly after Mr Trump had blocked the release of millions of dollars in US military aid to Ukraine. A senior official later testified that the president made clear the release of the aid was conditional on Mr Biden being investigated, but the White House denies this.
What is his defence?
Mr Trump denies using US military aid as a bargaining chip with Mr Zelensky and has repeatedly insisted his call with Ukraine's leader was "perfect".
He has called the impeachment inquiry a "witch hunt" by Democrats and elements of the media.
The Republican defence comes in three parts:
- Ukraine's president said he felt no pressure
- The Ukrainians were unaware the aid was held back
- US military aid was eventually released
What is impeachment anyway?
What is impeachment anyway?
In this context, to "impeach" means to bring charges in Congress which will form the basis for a trial.
To impeach, in this context, means to bring charges in Congress that will form the basis for a trial.
The US constitution states a president "shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes or misdemeanours".
The US constitution states a president "shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes or misdemeanours".
The process of impeachment has to be started by the House of Representatives and only needs a simple majority to pass. The trial will be held in the Senate.
What is the process?
But here, a two-thirds vote is necessary for removal - and this milestone has never been reached in America's history.
It happens in two stages. Proceedings have to be started by the House of Representatives.
Who has actually been impeached?
A vote to impeach only needs a simple majority to pass and if it does, the process then moves to the Senate where a trial is held.
Despite it being threatened on numerous occasions, only two presidents have ever actually been impeached.
But here, a two-thirds vote is necessary for a president's removal - and this milestone has never been reached in US history.
Most recently, Bill Clinton - the 42nd president of the United States - found himself impeached on the grounds of perjury in front of a grand jury 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.
The Senate is currently controlled by the Republican Party.
The House voted 228 to 206 in favour of impeaching President Clinton for the first charge, and 221 to 212 on the second.
Have other US presidents been impeached?
It should be noted that, at the time in December 1998, Mr Clinton's approval rating as president was at 72%.
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.
However, when the trial reached the Senate in 1999, it failed to get close to the two-thirds backing needed in order to pass. As an analysis piece the BBC ran at the time noted, "in their eagerness to bring down the president, they never stopped to think whether the charges could be proved beyond reasonable doubt".
But when the trial reached the Senate in 1999, the vote for a conviction failed to get close to the two-thirds backing required.
The second? Clue: It wasn't Richard Nixon. (More on this lower down.)
The only other president impeached was Andrew Johnson 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.
In fact, the only other president impeached was Andrew Johnson, who served for four years from 1865 - the 17th person to hold the role.
Richard Nixon, the 37th US president, resigned in 1974 before he could be impeached over the Watergate scandal.
He was impeached by the House in 1868. The vote came just 11 days after he got rid of Edwin Stanton, his secretary of war - a man who didn't agree with his policies.
Who would replace Trump?
The parallels between Mr Stanton's firing and that of FBI director James Comey - a man who disagreed with Mr Trump - did not go unnoticed in the American press.
The line of succession for the US government, as established by the Presidential Succession Act of 1947, would mean Vice-President Mike Pence moving into the Oval Office.
Unlike Mr Clinton, however, Mr Johnson's survival was a close call: the two-thirds majority was missed by just one vote, thanks to a number of Republicans.
Learn more about impeachment inquiry
Later, Iowa senator James Grimes explained: "I cannot agree to destroy the harmonious working of the Constitution for the sake of getting rid of an unacceptable president."
So, could Mr Trump be impeached?
Analysis - Anthony Zurcher, BBC News, Washington
For months now, Democratic leaders in the House of Representatives have been playing a semantics game. They wanted those who supported and those who opposed a formal impeachment inquiry into President Donald Trump to both think they were getting what they wanted.
This strategy suggested a fear by Speaker Nancy Pelosi and others that heading down the path to impeachment would put moderate Democrats facing tough 2020 re-election fights at risk.
That calculus appears to have changed, after the rapid drumbeat of new revelations about Mr Trump's contacts with Ukrainian President Volodymyr Zelensky. Now even middle-of-the road politicians are coming out in favour of impeachment proceedings.
The dam has broken. The genie is out of the bottle. Pick your metaphor. The simple fact is that Ms Pelosi - a keen judge of the political mood within her caucus - has made the decision to shift from resisting impeachment to -at the very least - being open to it.
The path forward is uncertain. The administration could back way from its across-the board stonewalling and give Congress some of the information it requests. Opinion surveys could show the latest drama is taking a toll on one party or the other, causing political will to crumble. Or, both sides could dig in for a long, gruelling battle that could drag into the darkest days of winter.
Say Trump IS impeached... would he be removed from office?
Republicans control the Senate, so Mr Trump would not be removed from office unless members of his own party turned on him.
The vast majority of Republicans have remained loyal to him.
Of course, there are the notable exceptions, such as Senator Mitt Romney who was a lone voice among Republican senators calling for more White House transparency over the Ukraine contacts.
But it looks like Mr Trump would remain in the White House, thanks to his support among Republicans.
In the wider public, the president remains unpopular but the appetite for impeachment is low.
A Monmouth University poll carried out this month - before the Ukraine story - suggests 35% of Americans feel Mr Trump should be impeached.
Finally - just how did Mr Nixon avoid impeachment?
He did what every sensible person does when they know the tide has turned against them. He quit.
