Updated on Jan. 20 at 11:15 p.m. ET

The Senate is beginning a trial on whether or not to remove President Trump from office, a little over a month after the House voted to impeach him.

Read the latest news about the inquiry; listen to our special broadcast coverage.

While momentum toward impeachment had been building among Democrats for months, House Speaker Nancy Pelosi, D-Calif., announced an inquiry in September — after a whistleblower complaint about a White House phone call with Ukraine.

In a call with Ukrainian President Volodymyr Zelenskiy on July 25, Trump asked for an investigation into a debunked conspiracy theory about the 2016 election and into potential 2020 rival and former Vice President Joe Biden. The White House is also accused of withholding military aid to Ukraine for Trump's personal political gain. Trump maintains that he has done nothing wrong.

The House conducted an inquiry that involved a series of closed-door depositions followed by open hearings with key witnesses, including career diplomats with expertise in Ukraine policy. The House Judiciary Committee then voted to recommend impeachment, and the full House voted to impeach on Dec. 18.

The Senate is voting on the rules of its trial on Jan. 21, and opening arguments are expected to begin the next day. At the end of the trial the Senate will vote on whether to remove Trump from office or acquit him.

The House brought two articles of impeachment against Trump: abuse of power and obstruction of Congress.

"President Trump has acted in a manner contrary to his trust as President and subversive of constitutional government, to the great prejudice of the cause of law and justice, and to the manifest injury of the people of the United States," the resolution reads.

Read the articles.

Following the House vote to impeach, the Senate holds a trial. Republican and Democratic leadership have been fighting over what constitutes a "fair" trial and whether that includes witnesses who did not take part in the House's investigation.

In the trial, members of the House serve as "managers" who make the case for Trump's removal. The president has his own defense team, including White House counsel, outside attorneys and Republican members of the House.

After the proceedings, senators vote on whether to remove or acquit Trump, based on each article.

Read more about the trial process.

The White House has not released a full list of participants on the July 25 call, but NPR has compiled a list of people identified so far. Here's who they are.

From Trump's early focus on Ukraine in 2017, to the release of the whistleblower complaint on Sept. 26, here's how we got to the impeachment inquiry.

Separately, here is a full timeline of the impeachment inquiry, including these key dates:

Dec. 11: House Judiciary begins debate on articles
Dec. 13: House Judiciary approves articles of impeachment
Dec. 18: Full House debates the articles and then votes to impeach
Jan. 16: House impeachment managers hand-deliver the articles to the Senate, a day after being appointed. U.S. Chief Justice John Roberts and senators are sworn in.
Jan. 21: Opening arguments begin in the Senate trial.

Since the original whistleblower complaint was released, the list of names of those connected to Trump's call with Zelenskiy or to broader Ukraine policy has grown substantially.

From the president to career diplomats to private lawyers, here is a quick guide to people connected to the events being investigated.

In-depth profiles and features: