US Supreme Court Chief Justice John Roberts has been sworn in to preside over the impeachment trial of President Donald Trump.
Senate members will serve as jurors in the case.
The judge administered the oath to the senators, who will decide whether or not the president should be removed from office.
Adam Schiff, chairman of the House Intelligence Committee who will serve as lead prosecutor for the trial, then read out the two articles of impeachment accusing Trump of "high crimes and misdemeanors."
The trial has been adjourned until Tuesday when opening statements are expected to be made.
Earlier, the articles of impeachment or charges against President Trump were read aloud on the floor of the Senate.
Four weeks ago, the Democratic-controlled House of Representatives impeached the president for abuse of power in his dealings with Ukraine and obstruction of Congress.
A pivotal event in the impeachment case against President Trump was a 25 July telephone call in which he asked Ukraine's president to open a corruption investigation into Joe Biden and his son, as well as a discredited theory that Ukraine, not Russia, interfered in the 2016 US election.
Democrats have called this an abuse of power because Mr Trump asked a foreign government to interfere in a US election for his own benefit at the expense of American national security.
Mr Biden, the former US vice president, is vying for the Democratic nomination to face Mr Trump in the presidential election.
Republicans have argued that Trump's actions did not rise to the level of impeachable offences.
They have accused Democrats of using the Ukraine affair to try to nullify Mr Trump's 2016 election victory.
Democrats also accused Mr Trump of abusing his power by withholding $391 million in security aid to Ukraine as leverage to pressure Kiev into conducting investigations politically beneficial to him.
The money - approved by Congress to help Ukraine combat Russia-backed separatists - eventually was provided to Ukraine in September after the controversy became public.
