The new Pope will lead his first holy mass since being installed as head of the Catholic Church this morning.
Robert Prevost - now known as Leo the 14th - will be joined by cardinals in the Sistine Chapel.
His surprise election came yesterday - with white smoke billowing above the Vatican after just four rounds of votes.
He's the first ever American pontiff - and in Chicago, his brother Louis is celebrating.
"I just freaked out and said it's Rob, we kind of knew he was special and we used to tease him about being Pope when he was 6 years old."
