They're known to have a playful relationship.
While Prince Harry teases his brother over his thinning hair, having once joked that he was "already bald aged 12", William has often got his own back by calling him "a ginger".
So for Harry's best man speech at the upcoming royal nuptials, it's no surprise that he plans on continuing in the same vein.
Along with a couple of the groom's friends, the 24-year-old intends to "dig him in the ribs and embarrass him. Make him lose some hair," he told the BBC.
"The speech will be like any best man's speech, between the three of us we will make sure people know the good times and bad times since he was a nipper."
But the 26-year-old also acknowledged that he will have to be careful due to his grandmother, the Queen's, presence.
"I will have a few stories but my grandmother will be there so I will have to, er, be selective."
He hasn't revealed who his co-conspirators are, but it has been suggested that they will be Thomas van Straubenzee and James Meade, William's close friends from school.