US soldiers are leaving towns and cities in Iraq, six years after they invaded the country.
A public holiday - National Sovereignty Day - has been declared, and the capital, Baghdad, threw a giant party to mark the eve of the changeover.
But US soldiers are staying in the country for another two years, to support Iraqi security forces.
US troop numbers in Iraq won't start to fall until after elections there in January next year.
American soldiers invaded Iraq with a number of other countries in 2003.
They were officially looking for dangerous weapons that they said the man who was in charge of Iraq, Saddam Hussein, had built.
But those weapons have never been found, and lots of people say they never existed.
Since troops invaded, the country has seen a lot of violence and trouble.
But the situation has been improving, and now US forces are handing over responsibility for security to Iraqi forces.