The Democratic-led US House of Representatives will hold on charges of abuse of power and obstruction of Congress.
A vote for impeachment will lead to a trial in the Republican-controlled Senate, where politicians must decide whether to convict Trump and remove him from office.
Here is what's happened so far and what's still to come:
Activists gather at a 'Impeach and Remove' rally to show support for the Congressional impeachment of US President Donald Trump. Source: EPA
Wednesday, 18 December
The House convened at 9am local time (0100 Thursday AEDT) and spent an hour debating a rule setting the parameters for a six-hour debate on the articles of impeachment.
Legislators are now debating the articles.
Once the debate ends, probably in late afternoon or early evening (Thursday AEST) the 431 sitting House legislators will hold separate votes on the two articles of impeachment.
The vote is likely to fall almost entirely along party lines, with support from nearly all of the chamber's 233 Democrats. All 197 House Republicans are expected to vote no.
If impeachment is approved, the House would select politicians known as managers to present the case against Mr Trump at a Senate trial. House Democrats say most of the managers are likely to come from the Judiciary Committee, and possibly from the Intelligence Committee that led the investigation.
Early January
Mr Trump would face a trial in the Senate to determine whether he should be convicted and ousted from office. The Senate is controlled by Mr Trump's fellow Republicans, who have shown little sign they will find him guilty. A two-thirds majority of those present in the 100-member chamber would be needed to convict Mr Trump.
US Chief Justice John Roberts would preside over the trial. House managers would present their case against Mr Trump, and the president's legal team would respond. Senators would act as jurors. A trial could involve testimony from witnesses and a gruelling schedule in which proceedings occur six days a week for as many as six weeks.
Senate Majority Leader Mitch McConnell has said a majority of the Senate could approve a shorter process by voting on the articles of impeachment after opening arguments, without witnesses.
