We mentioned the applause earlier for Vindman's patriotic words.

You can see the clip yourself in the C-Span tweet below...

Vindman has requested a security assessment to analyse his and his family's safety, US media reports.

The US army is reportedly prepared to move Vindman and his family to a secure location on a military base if they are found to be in danger due to his testimony today.

"The Army is providing supportive assistance to help Lt Col Vindman with the public attention," an Army spokesperson told USA Today.

"As a matter of practice, the Army would neither confirm or deny any safety or security measures... however, as we would with any solider, the Army will work with civilian authorities to ensure he and his family are properly protected."

Congressman Joaquin Castro, whose twin brother Julian Castro is running to be US president, offered a joke to Vindman whose own twin brother is also in government and attended today's hearing.

Kurt Volker is the former special representative to Ukraine - and one of the key players in the alleged efforts to pressure Ukraine by withholding military aid.

He apparently worked with Ambassador Gordon Sondland and Energy Secretary Rick Perry on Ukraine policy through the "irregular" diplomatic channel.

In closed-door testimony in October, Volker told Congress he was “never asked to do anything” he thought was wrong, but added that he worried the US-Ukraine relationship was “getting sucked into a domestic political debate”.

Volker has testified that he knew what Trump lawyer Rudy Giuliani was doing in Ukraine, pushing unsubstantiated claims against the Bidens.

Image source, Getty Images

White House spokeswoman Stephanie Grisham has just sent out this statement.

"We have learned nothing new in today’s illegitimate ‘impeachment’ proceedings.

"However, buried among the witnesses’ personal opinions and conjecture about a call the White House long ago released to the public, both witnesses testified the July 25 transcript was ‘accurate’ and nothing President Trump has done or said amounts to ‘bribery’ or any other crime.

"Today’s hearing only further exposes that Chairman Schiff and the Democrats are simply blinded by their hatred for Donald Trump and rabid desire to overturn the outcome of a free and fair election."

Image source, Getty Images

As the hearing takes a lunch break, let's take a look at who is due to testify this afternoon.

Tim Morrison could be instrumental in tying the so-called “irregular” shadow foreign policy team in Ukraine directly to Trump.

The former top Russia and Europe expert on the National Security Council - who listened in on the Trump-Zelensky call - quit one day before delivering closed-door testimony to the intelligence committee last month.

In his deposition, Morrison told lawmakers that EU Ambassador Gordon Sondland had told him that Sondland was acting on orders from Trump, and that Trump and Sondland had spoken at least six times.

“His mandate from the president was to go make deals,” Morrison said of Sondland.

He added that Sondland had told him that he informed the Ukrainians that they would receive withheld US military assistance if Zelsenky announced an investigation of the Bidens.

He also told lawmakers that he was concerned by the Trump-Zelensky call and reported it up his chain of command, before asking that the call be restricted due to its potential for national security damage.

A progressive veterans' group has condemned attacks on Vindman's character.

Vindman was clapped in the committee room for his comments about why he's taking on the president.

"Why are you confident that you can tell your dad not to worry about you challenging President Trump?" asks New York Democrat Sean Maloney.

"This is America," Vindman says. "Here, right matters."

The White House, meanwhile, hits back at Vindman, who is still one of its employees.

The cameras are fixated on the witnesses and their interrogators but there are plenty more people in the room and vicinity.

Here's a glimpse at what else is happening on Capitol Hill.

Image source, Jacquelyn Martin-Pool/Getty Images

Image source, OLIVIER DOULIERY/AFP via Getty Images

Image source, Drew Angerer/Getty Images

Image source, Getty Images

Image source, Jacquelyn Martin - Pool/Getty Images

Image source, Sarah Silbiger/Getty Images

Image source, ANDREW CABALLERO-REYNOLDS/POOL/AFP via Getty Images

Anthony Zurcher
BBC North America reporter

Image source, Shawn Thew - Pool/Getty Images

If what Vindman said was important, how he said it in the public hearings also mattered.

Behind closed doors, veteran ambassadors Bill Taylor and Marie Yovanovitch were reportedly smooth while Vindman was halting and nervous. Those observations have been confirmed by their public testimony.

When Vindman delivered his opening statement, a few yards from where I was seated, his hands trembled slightly. He occasionally stumbled on his words.

Republicans could paint that as weakness or uncertainty, but it might also be seen by Americans as giving his testimony a touch of humanity - particularly when paired with the emotional closing words to Vindman’s opening statement.

He offered reassurance to his father, who brought his children to the US from the Soviet Union 40 years ago, that he was sitting in the US Capitol and would be “fine for telling the truth”.

Perhaps, but Vindman’s testimony will only add to the contentious debate among Democrats and Republicans over exactly what the truth is.

Image source, Jacquelyn Martin - Pool/Getty Images

Republican Congresswoman Elise Stefanik turned her questioning to Hunter Biden, saying her constituents have "many concerns" about the ex-vice-president's son's former position on the board of Burisma.

The New York congresswoman asks both Williams and Vindman if Biden's presence on the board had the "potential" for the "appearance" of a conflict of interest. Both agree.

Stefanik - once the youngest woman ever elected to Congress - has emerged as a rising star in the midst of the Democratic-led inquiry.

The 35-year-old squared off against ranking Democrat Adam Schiff during the hearings last week, accusing him of trying to silence her and others in her party, "simply because we are Republicans".

Her performance has drawn widespread praise from Conservatives, including President Trump, who called her a "new Republican star" on Twitter.

But Stefanik's strength has also energised her opponents. Tedra Cobb - Stefanik's 2020 opponent - reported receiving more than $1 million in campaign donations this past weekend alone.

Democrat Adam Schiff weighs in briefly to clarify bribery following Republican John Ratcliffe's statements.

Mr Schiff says bribery can involve " a conditioning of a specific act for something of value".

"An official act may be a White House meeting. An official act may be $400m in military aid and something of value."

Schiff explains that the reason lawmakers don't ask "fact witnesses" to make a judgment call on bribery is because the Founding Fathers indicated in the US Constitution that Congress should make that determination.

"It will be our job to decide whether the impeachable act of bribery has occurred. that's why we don't ask you those questions."

Image source, Drew Angerer/Getty Images

Utah Republican Chris Stewart, also from a military family, questions Vindman's testimony that Trump's request for Zelensky to investigate the Bidens amounted to a "demand".

"When a superior makes a request that's an order," Vindman says.

Stewart says Vindman is defining a demand "based on your military experience".

The lawmaker points out that Trump and Zelensky have no such military background.

"Would it be fair to take a person who has never served in the military and take a military evaluation of those words and attach them to somebody who has never served?"

Vindman says: "The context of this call, consistent with the 10 July meeting, with the reporting that was going on including the president's personal attorney made clear that this was not simply a request."

Stewart says: "I think it's nonsense."

Image source, Chip Somodevilla/Getty Images

Democrat Jackie Speier asks whether Vindman has noticed any White House reprisals against him for testifying to the impeachment hearings.

He says: "I did notice I was being excluded from several meetings that would have been appropriate for my position." But he does not explicitly characterise that as reprisals per se.

Williams says she has not noticed any such retaliation against her.

Trump has been speaking from the White House.

Here's what he said, after watching "a little bit" of the hearing this morning:

"I don't know him [Vindman]... I never saw the man. I understand now he wears his uniform when he goes in. No, I don't know Vindman at all."

He added: "I don't know any of these people," referring to other impeachment witnesses who testified last week.

He also called Democratic Speaker of the House Nancy Pelosi "grossly incompetent" and praised Republicans for "absolutely killing it" in the hearings.

Image source, Google Trends

Google searches for "impeachment" in the US over the past day peaked at 09:00 this morning, just as Williams and Vindman were sworn in to testify.

Interest was highest in Washington, DC, not a surprise as the impeachment probe has obsessed the nation's capital.

Americans in the south seem less interested: Alabama, Louisiana and Mississippi had the lowest search rates in the country.

Ohio Republican Mike Turner asks Vindman if he has ever met or spoken to President Trump.

Vindman says no.

Turner hits back: "You do know this impeachment inquiry is about the President of the United States?"

"Yes," Vindman says.

Turner also questions the witness' reference to himself as co-ordinator for Ukraine policy, noting that the secretary of state and other officials do not report to Vindman.

Vindman says he was in charge of facilitating co-operation between agencies about Ukraine policy, not that he formulated such policy.

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

FACT-CHECK: Is the whistleblower linked to Democrats?

CONTEXT: Why Ukraine matters to the US