The investigation, looking at the whole period from 2001 to 2009, is expected to last months, with a report not out until after the next general election.
Former Prime Minister Tony Blair will be among the future witnesses.
Tuesday's session looks at UK foreign policy towards Iraq in the lead-up to the war, which began in 2003.
The Iraq inquiry will begin with a statement from its chairman, Sir John Chilcot.
It will then hear from figures including Sir Peter Ricketts, who was the chairman of the Joint Intelligence Committee (JIC) - which oversees MI5, MI6 and GCHQ - from 2000 to 2001.
Others giving evidence are former senior Ministry of Defence official Simon Webb and ex-Foreign Office officials Sir Michael Wood and Sir William Patey.
Controversial dossier
The members of the committee were chosen by Downing Street, leading critics to ask whether it can be independent of the government.
But Sir John has promised the inquiry will not produce a "whitewash".
On Wednesday, the panel will hear from former senior Foreign Office staff on the claims that Saddam Hussein's regime possessed "weapons of mass destruction".
Over the coming weeks the inquiry is expected to hear from a succession of diplomats, military officers and politicians, including Mr Blair, who is due to appear early in the new year.
Sir John Scarlett, the former chief of MI6 who - as chairman of the JIC - drew up the Government's controversial dossier on Iraq's weapons of mass destruction, is also due to give evidence to the five-strong inquiry panel.
The war resulted in the deaths of 179 UK forces personnel.
Previously, the Butler inquiry looked at intelligence failures before the war, while the Hutton inquiry examined the circumstances leading to the death of former government adviser David Kelly.
Sir John Chilcot has said he hopes to complete his final report by the end of next year, although he has warned it could slip into 2011.
