Prince William went to university after finishing school. He then joined the Army and trained to be an officer at Sandhurst.
His dad, uncle and granddad, Prince Charles, Prince Andrew and Prince Philip, have all served in the Royal Navy.
After about a year in the Army, William spent time with both the RAF and the Royal Navy.
He is currently an RAF Search and Rescue Pilot.
William is expected to be king at some point in his life, and when he is he will be the head of the armed forces.