Comey has alleged in memos that Trump asked him to pledge loyalty and to shut down his inquiry into former national security adviser Michael Flynn. Even if Comey can't talk about sensitive details of an ongoing criminal investigation, legislators will likely want to inquire about his conversations with the president and the circumstances of his firing.
Are there any limits to what Comey can say?
Investigators - led by newly appointed special counsel Robert Mueller III - are still working on the Russia case. Even before Mueller was appointed, Comey would almost certainly not have been willing to discuss the details of their work. That is still the case now that Mueller is in charge.
People familiar with the matter, though, say Comey has reached an understanding with Mueller's office about what he can and cannot discuss, and it seems Comey will be allowed to reveal at least some details of his exchanges with the President.
While Comey's memos on those exchanges already have been reported on, Comey could offer more detail and context. And his testimony would be the first time he has publicly commented. So far, the memos have only been described to reporters by people who have seen them or been briefed on their contents.
Can the White House stop this?
The President could possibly invoke some form of executive privilege - which is designed to keep sensitive decision-making processes involving the President secret - to stop Comey from testifying. The Administration had discussions about privilege with former acting Attorney General Sally Yates ahead of her testifying before the Senate Judiciary Committee on Russia and other matters last month.
In that case, though, Yates ultimately was allowed to testify, and the White House might be on even shakier footing to try to block Comey. The President, after all, has tweeted extensively about the Russia investigation and even mentioned conversations with Comey in his letter firing the FBI director. Trump claimed in that missive that Comey had told him three times he was not under investigation.
If the White House did block Comey, it would also spark a political outcry and perhaps damage Trump even more than any unflattering testimony.
Will this affect the Russia investigation?
Comey could say something that is of interest to the new special counsel, particularly on the question of whether justice was obstructed. But given that he has already been in touch with Mueller's office, that seems unlikely.
That's not to say the hearing won't be interesting.
Comey is a gifted orator who is not shy about claiming the spotlight. In 2007, he famously described to lawmakers how he raced to the hospital room of Attorney General John Ashcroft to stop, at least for a while, the approval of a National Security Agency surveillance programme that he believed to be unlawful. The intervention has become lore in Washington and helped foster Comey's reputation as a fiercely independent official, unafraid to stand in the way of perceived wrongdoing and then expose it publicly.
