What to expect later todaypublished at 12:31 Greenwich Mean Time 9 February 2021
No other US president has ever been impeached twice and no president has gone on trial after leaving office - so we are in pretty uncharted territory.
The Senate, which is made up of 100 elected members, convenes today at 13:00 EST (18:00 GMT) and the trial will begin with a debate and vote on whether the trial is unconstitutional.
Opening arguments are expected to begin on Wednesday.
Mr Trump, who has been at his Florida resort since leaving office, will not testify, his lawyers have said.
Millions watched the first impeachment of Trump, back in 2019, and his second in January, and this trial starting on Tuesday promises to be another blockbuster.