As children growing up in the "Little Odessa" neighbourhood of Brooklyn, the Vindman twins were interviewed by documentarian Ken Burns.

Hear what they said about emigrating to America.

Image source, Getty Images

Lt Col Alexander Vindman's identical twin brother, Lt Col Yevgeny Vindman, is also at the hearing.

Yevgeny Vindman is a lawyer working with the White House National Security Council, just like his brother.

Image source, Jim Lo Scalzo-Pool/Getty Images

Adam Schiff, top Democrat on the Intelligence Committee, has begun today's hearing.

In his opening statement, Schiff criticised President Trump's alleged corruption, saying the president "put his own political and personal interests" ahead of the country's.

Image source, Getty Images

Also on the eve of the hearings, Trump's first Secretary of State, Rex Tillerson, said that seeking political help from a foreign nation is "wrong".

Rex Tillerson told PBS on Monday: "If you’re seeking some kind of personal gain and you're using - whether it's American foreign aid or American weapons or American influence - that's wrong.

"And I think everyone understands that."

The former top diplomat and oil executive also said he thinks "the country is in good hands with the American people".

Tillerson has previously criticised Trump as "a man who is pretty undisciplined, doesn’t like to read, doesn’t read briefing reports, doesn’t like to get into the details of a lot of things".

Trump, in turn, called him "dumb as a rock" and "lazy as hell".

Image source, Justin Sullivan/Getty Images

On the eve of Vindman's testimony, Republican Senator Ron Johnson issued an 11-page letter to top Republicans, suggesting the Purple Heart recipient may have participated "in the ongoing effort to sabotage" Trump's policies "and if possible, remove him from office".

Johnson's attack may preview the Republican strategy to undermine Vindman as a witness today.

“I believe that a significant number of bureaucrats and staff members within the executive branch have never accepted President Trump as legitimate and resent his unorthodox style," the Wisconsin senator wrote.

"It is entirely possible Vindman fits this profile."

Johnson himself has had a role in key moments relating to Ukraine this year, attending President Volodymyr Zelensky's inauguration in May.

Here are your handy primers for all things impeachment.

SIMPLE GUIDE: If you want a basic take on what's going on, this one's for you

GO DEEPER: Here's a 100, 300 and 800-word summary of the story

WHAT'S IMPEACHMENT? It's a political process to remove a president - video guide

VIEW FROM TRUMP COUNTRY: Hear from residents in Pennsylvania

ON THE DOORSTEP: A newly elected Democrat sells impeachment to voters

Morrison is the former top Russia and Europe expert on the National Security Council. He was also authorised to listen in on the Trump-Zelensky call.

He quit a day before delivering closed-door testimony to the House intelligence committee last month.

"I want to be clear, I was not concerned that anything illegal was discussed," he told lawmakers.

He also criticised Vindman for taking his concerns about the Trump-Zelensky call directly to a White House lawyer, rather than through the chain of command.

In his deposition, Morrison said the US envoy to the EU, Gordon Sondland, had told the Ukrainians they would receive withheld US military assistance if they announced an investigation into the Bidens.

Morrison testified that Sondland had spoken to Trump about half a dozen times over the summer as the White House froze aid to Ukraine. (Trump has said of Sondland: "I hardly know the gentleman.")

Morrison also said Sondland told him he was acting at Trump's request.

Kurt Volker is the former special representative to Ukraine.

In closed-door testimony in October, Volker told lawmakers he was "never asked to do anything" he thought was wrong - something Republicans will surely highlight today.

But Volker also said he worried the US-Ukraine relationship was "getting sucked into a domestic political debate".

Text messages Volker provided to Congress show he spoke to Ukrainian officials about launching politically charged investigations.

He was one of the self-titled "three amigos" who were charged by the White House with guiding US-Ukraine relations - the other two were EU ambassador Gordon Sondland and Energy Secretary Rick Perry.

Volker resigned after the release of the whistleblower report about the July phone call between Trump and Ukraine's leader.

Jennifer WIlliams is US Vice-President Mike Pence’s top adviser on Russia.

She also heard the 25 July call between Trump and his Ukrainian counterpart.

In her private testimony earlier in November, Williams said the call had struck her as "unusual and inappropriate".

"I found the specific references to be more specific to the president in nature, to his personal political agenda… as opposed to a broader foreign policy objective of the United States."

Trump has also dismissed Williams as a "Never Trumper".

Pence’s office has sought to distance the vice-president from Williams in recent weeks, declining to defend her from Trump’s attacks and saying: "Jennifer is a State Department employee."

Who is Lt Col Alexander Vindman?

Lt Col Vindman, a 44-year-old Purple Heart recipient and Iraq War veteran, continues to serve in the White House as a Ukraine specialist within the National Security Council.

He was among officials who were authorised to listen in on Trump's 25 July call with newly elected Ukrainian President Volodymyr Zelensky, which is central to the impeachment inquiry.

In closed-door testimony last month, he told House of Representatives lawmakers he believed the president had crossed a "disturbing" line when he asked the Ukrainian president for a "favour" during that call.

Vindman said he was so alarmed that within an hour he told White House lawyers that the president’s actions were improper.

The president has dismissed him as a "Never Trumper".

Conservative media voices have cast aspersions on Vindman’s loyalty. He came to the US aged three with his family, who were refugees from Ukraine.

Read more on Vindman's past testimony here

Image source, Getty Images

Welcome to our live coverage as we enter the second week of public impeachment hearings into President Donald Trump.

Today, lawmakers will hear in back-to-back sessions from four witnesses about whether President Trump abused his power by pressuring Ukraine to investigate a political rival, Joe Biden.

First up are two White House officials: Ukraine expert Lt Col Alexander Vindman and Jennifer Williams, an aide to Vice-President Mike Pence.

In the afternoon, former US special envoy to Ukraine Kurt Volker and Tim Morrison, a former top Russia and Europe adviser on the National Security Council will appear together.