Here, we take a look at the circumstances of the US House of Representatives' vote to formally charge Mr Trump.
– What is impeachment?
Impeachment is the US constitution's ultimate remedy for high crimes and misdemeanours. It is a process by which charges are brought in the US Congress which will form the basis of a trial in the Senate.
The process is a political one, rather than criminal.
– What is Donald Trump accused of?
The president faces two charges – one that he abused the power of his office by enlisting a foreign government to investigate a political rival ahead of the 2020 presidential elections.
The second charge alleges that he obstructed US Congress in its investigation.
– How did the allegations come about?
The charges stem from a phone call in July, when Mr Trump is alleged to have asked the new Ukrainian president for a favour. Mr Trump is said to have sought information on senior Democrats, including the man widely tipped to be facing him in the 2020 elections: Joe Biden.
The president was allegedly seeking information on Mr Biden and his son, Hunter, the latter of whom worked on the board of a Ukrainian gas company while his father was vice president to Barack Obama. The allegations made during Mr Trump's call against Mr Biden and his son have since been discredited.
Mr Trump is then alleged to have used military aid and the promise of a White House meeting as leverage. Democrats say this is an abuse of power and a way of using his political office for personal gain at the expense of national security.
The Ukrainian president, Volodymyr Zelenskiy, was new to politics and government at the time. The former comedian was seeking a coveted White House visit to show backing from the US as he confronted a hostile Russia at his border.
Mr Zelenskiy was also counting on 391 million dollars (£298 million) in military aid already approved by US Congress. The White House had delayed the funds, but Mr Trump eventually released the money once Congress intervened.
– What about the whistleblower?
The vote in the US House of Representatives was sparked by a complaint from an anonymous whistleblower.
The intelligence officer wrote a letter of complaint expressing concerns about the telephone call on July 25 this year.
This led to two months of investigations leading to the US house intelligence committee's impeachment report, released earlier this month.
– What do the Democrats say?
The US house speaker, Nancy Pelosi, called the move "a great day for the constitution of the United States, a sad one for America that the president's reckless activities necessitated us having to introduce articles of impeachment".
Many other Democrats said they were carrying out their duty to protect the constitution and uphold the nation's system of checks and balances.
– How about Mr Trump and his fellow Republicans?
The US president has remained defiant, calling the whole affair a "witch hunt", a "hoax" and a "sham". He has described his phone call with his Ukrainian counterpart as "perfect" and insists he has done nothing wrong. In a letter to Ms Pelosi, Mr Trump compared his travails with the Salem witch trials.
Republicans have been in broad agreement, with many saying that Democrats are impeaching Mr Trump because they cannot beat him at the polls next year.
– What happens next?
After the US House of Representatives voted to impeach, this means that Mr Trump will face a trial in the Senate – the upper chamber of Congress.
This is expected to take place in January.
After the trial, the senate will vote on whether to convict the president.
In order to remove Mr Trump from office, the senate will have to vote in favour by a two-thirds majority.
The US House of Representatives is led by the Democrats – but the Republicans control the Senate. This means that it is highly unlikely Mr Trump will be convicted.
If Mr Trump is removed from office, vice president Mike Pence will take over at the White House, ahead of the presidential elections in November 2020.
SUCH ATROCIOUS LIES BY THE RADICAL LEFT, DO NOTHING DEMOCRATS. THIS IS AN ASSAULT ON AMERICA, AND AN ASSAULT ON THE REPUBLICAN PARTY!!!!
— Donald J. Trump (@realDonaldTrump) December 18, 2019
– Who else has been impeached?
Only two other presidents have faced the process in American history – Andrew Johnson and Bill Clinton.
In 1868, Mr Johnson faced a number of allegations, including a charge that he dismissed his secretary of war against the wishes of Congress. The Democrat narrowly survived his impeachment trial, with the two-thirds majority required to convict him being missed by just one vote.
Bill Clinton's impeachment in 1998 will be fresh in the minds of many. The Democratic president was accused of perjury and obstruction of justice when it was alleged he lied about the nature of his affair with intern Monica Lewinsky, and then also asked her to lie about it.
The vote for a conviction fell far short of the two-thirds majority required in 1999.
Richard Nixon resigned over the Watergate scandal in 1974 before he could be impeached.