They have three children – George, Charlotte and Louis – and are currently facing pregnancy speculation again.
Kate and William had known each other for more than 10 years before tying the knot at Westminster Abbey on April 29 2011.
But before walking down the aisle, the soon-to-be-Duchess said some poignant words to her father.
According to royal author Katie Nicholl, who covered the behind the scenes of the big day for Vanity Fair in 2011, her father looked like he was getting emotional.
Ms Nicholl said: “When her father looked as though he might falter on their journey to the Abbey, Kate had begged, ‘Please, Daddy, don’t cry. It will set me off, too.”
The author added: “She had managed only a bite of toast that morning and had been up since six, but Kate did not once let her emotion get the better of her.”
At the front of the Abbey, Harry was standing by his brother’s side, and couldn’t resist a peek.
Ms Nicholl explained: “She’s here,’ he said with a grin. ‘Just wait till you see her.’”
Fans were also desperate to hear what William said to Kate when she arrived by his side.
Royal author Marcia Moody explained exactly what he said in her 2013 book Kate: A Biography.
Ms Moody writes: “The couple set eyes on each other, and William murmured to Kate, ‘you look beautiful’.”
William also turned to the father of the bride and joked: “Just a quiet family wedding?”
Ms Moody continued: “Michael [Middleton] later said he wasn’t nervous because he had Kate on his arm, and even told friends it felt like a dream until he opened the newspapers the following day.”