US President Donald Trump's efforts to pressure Ukraine into investigating a top rival in the 2020 US presidential election prompted Speaker Nancy Pelosi and other Democrats in the House of Representatives to move ahead with an impeachment inquiry, threatening his presidency.
The scandal has also featured an extensive cast of Ukrainian officials.
Here are some of the other main American players in the drama.
Trump's personal attorney, a former New York mayor and 2008 presidential candidate, is a central figure in a scandal that has led to the impeachment inquiry.
Acting independently of the US government, Giuliani has pressed Ukrainian officials to investigate one of Trump's main rivals in the 2020 presidential election, former Vice President Joe Biden.
Giuliani says Biden improperly pressured Ukraine to fire its top prosecutor, Viktor Shokin, in order to stop an investigation into a company that put his son, Hunter, on its board.
In a July 25 phone call, Trump urged Ukraine President Volodymyr Zelenskiy to meet with Giuliani and US Attorney General William Barr to discuss the president's concerns about Biden.
The Democratic-led House Intelligence Committee has given Giuliani until Oct. 15 to turn over documents related to his work.
The former vice president has long led a crowded field of 19 candidates hoping to win the Democratic Party's nomination to face Trump in the November 2020 election.
Biden has accused Giuliani of peddling "false, debunked conspiracy theories" and has urged US television networks to stop booking him to appear on air.
As vice president under Trump's predecessor, Democratic President Barack Obama, Biden played a leading role in US efforts to dismiss Shokin over corruption concerns. Biden says he told Ukrainian President Petro Poroshenko in Kiev in December 2015 that the United States would withhold $1 billion in loan guarantees if Shokin remained on the job.
Biden's son served on the board of Burisma Group, a privately held natural gas producer, between 2014 and 2018. Critics have suggested he was not doing actual work for the company, an allegation he denies.
Burisma has faced allegations of dodging taxes and improperly securing licenses for gas deposits.
Ukrainian officials announced on Oct. 4 that they would review 15 previous investigations related to Burisma, and the country's National Anti-Corruption Bureau said in September it was investigating activity at Burisma between 2010 and 2012, prior to Hunter Biden's involvement. The company denies any wrongdoing.
Ukraine's top prosecutor, Ruslan Ryaboshapka, told Reuters he was not aware of any evidence of wrongdoing by Hunter Biden.
In the July 25 phone call, Trump encouraged Zelenskiy to work with Barr, the US attorney general, to investigate Joe Biden and his son. Barr didn't learn of the call until several weeks after it took place, and hasn't communicated with Ukraine, according to the Justice Department.
But in his nine months as the nation's top law enforcement official, Barr has taken several actions that have benefited Trump and caused Democrats to question his independence. He decided in March not to prosecute Trump for obstruction of justice for interfering with Special Counsel Robert Mueller's investigation of Russian attempts to influence the 2016 election.
When he released Mueller's report in April, Barr echoed Trump's refrain that the special counsel had found "no collusion" between his campaign and Russia. Mueller in fact had concluded that there was not enough evidence to bring criminal conspiracy charges, but outlined numerous examples where campaign officials welcomed Russian interference in the election.
In May, Barr announced an investigation into the origins of the Mueller probe, acting on accusations by Trump's political allies that FBI and intelligence officials were trying to undermine his political prospects. Barr has enlisted Trump to encourage foreign governments to cooperate with the investigation and, according to media reports, has traveled internationally to meet with foreign officials on the matter.
The US secretary of state, a vocal Trump supporter, was on the July 25 phone call between Trump and Zelenskiy but did not say whether he heard anything unusual. He has resisted Democrats' efforts to obtain depositions of current and former State Department officials as part of their impeachment inquiry.
House Foreign Affairs Committee Chairman Eliot Engel says Pompeo is a "fact witness" and has accused him of stonewalling their inquiry.
Before becoming the United States' top diplomat, Pompeo was Trump's CIA chief and before that a Republican congressman.
A former US special representative for Ukraine, Volker was one of two American diplomats who pressured their Ukrainian counterparts to launch investigations that could benefit Trump, according to text messages he provided to Congress. Volker worked with Giuliani on the effort, the messages show.
Those communications show he told a Ukrainian official that a meeting between the countries' two leaders was tied to Kiev's agreement to investigate the 2016 US election. He later pressured Ukraine to issue a public statement announcing probes into Burisma and the 2016 election.
Volker resigned his post in late September.
Sondland, the US ambassador to the European Union, was also involved in the effort to get Ukraine to investigate Biden and the 2016 election, the text messages show.
In one exchange, Sondland sought to quell concerns by another US ambassador, Bill Taylor - who as charge d'affaires in Ukraine is the senior diplomat there - that the Trump administration was withholding foreign aid until Kiev agreed to help with Trump's political campaign, and suggested they "stop the back and forth by text."
The former US ambassador to Ukraine was abruptly recalled to the United States in May, leaving the United States without a top-level diplomatic presence in a country on the front lines of the West's standoff with Russia.
A career diplomat with the US State Department who had served under Republican and Democratic administrations, Yovanovitch had been subjected to attacks by Trump supporters. Giuliani said she had been removed because she was insufficiently loyal to Trump. Democrats called her departure politically motivated.
Trump called her "bad news" in his July phone call with Zelinskiy.
As the Intelligence Community's inspector general, Atkinson serves as the internal watchdog for intelligence agencies like the director of national intelligence and the CIA. In that role, he handled a complaint by an unidentified whistleblower who first reported concerns about the Trump administration's efforts to improperly involve Ukraine in US affairs. Atkinson deemed the complaint credible, and decided that it qualified as an "urgent concern" which by law should be shared with Congress.
However, the Justice Department and the acting director of national intelligence, Joseph Maguire, determined that the complaint should not be shared with lawmakers. Atkinson then notified Congress of the dispute, which ultimately led to both the complaint and the summary of Trump's July 25 call being released.
Atkinson said he will fight to protect the identity of the whistleblower, who has been the subject of attacks by Trump.
Before signing on as inspector general in May 2018, Atkinson worked as a federal prosecutor specializing in corruption cases.