Former Prime Minister Tony Blair is explaining why he took Britain to war against Iraq.
It's all part of a massive inquiry into the war which started when Mr Blair was leader.
The people in charge of the inquiry are also trying to find out whether British troops were properly prepared to fight, and what's happened in Iraq since.
Lots of protesters who think going to war was wrong have gathered outside the building where the inquiry is going on.
The war began when America led an invasion of Iraq in 2003. British and American leaders thought that the country was dangerous and might attack them.
Britain left Iraq in 2009, but some American soldiers are still there. A total of 179 British troops died in the fighting.
Mr Blair will answer questions for around six hours on Friday.
Prime Minister Gordon Brown is also set to face the inquiry in the next few weeks.
Because there's so much information to go through, the report into the war might not be finished until 2011.