It's so important that people have been queuing overnight to watch Mr Blair give evidence, and anti-war protesters have started a demonstration.
He starts giving evidence at 1030 at the Royal Courts of Justice in London, and will face lots of tough questions.
The Inquiry wants to know if Mr Blair asked for a report into Iraq's weapons to be changed to make Saddam Hussein seem more dangerous than he actually was.
Was he responsible?
Mr Blair will also be asked whether he had anything to do with releasing Dr Kelly's name to the press, which many people think led to his death.
A lot of important people have appeared in front of the Inquiry, including Defence Secretary Geoff Hoon, and the BBC journalists who spoke to Dr Kelly.
Hear story
Should UK troops still be in Iraq?
Test your knowledge of world leaders
