Kate's every move will be scrutinized -- just like it is now -- and her image will become public property.
"I know that I'm very fortunate," said Prince William when the couple were in Lancashire Monday. "I have the support of my family and friends, I do a job I love, and I have Catherine."
It's a huge jump for someone who's not accustomed to a public life.
But Kate's stepmother-in-law, Camilla, is providing friendship and training to the newest royal.
"I think there's almost a maternal bond between Camilla and ... Catherine," ABC News' royal contributor Duncan Larcombe said. "There's a genuine friendship there."
When William was away on his recent trip to Australia, Camilla and Charles invited Kate to the ballet.
Also, Kate and Camilla were recently spotted lunching together. It was a very long lunch and full of wedding discussions.
A fellow diner heard someone say: "We can't have trumpets; the ceiling is too high."
"Camilla obviously knows what it's like to be thrown into the spotlight," says Larcombe. "I mean, who better, frankly, to get advice from than Camilla."
Camilla has seen it from all sides. She was once reviled as the woman who broke up Charles and Diana's marriage, and was dubbed the rottweiler.
But Camilla has come out the other side and has won the public's affection.
Sometimes Camilla's reactions surprise everyone. For example, her reaction to the engagement:
"It's the most brilliant news," said Camilla. "I'm just so happy for both of them. They are so happy. It's wicked!"
Prince Charles, we hear, is also very fond of Kate.
"I was quite nervous about meeting William's father," said Kate, "but he was very, very welcoming, very friendly. It couldn't have gone easier ... for me."
And Charles has been helping Kate with the wedding plans.
"You know, he's over the moon, actually," said little brother Harry when interviewed by ABC News on his recent trek to the North Pole.
"He's had a lot to do with the wedding, which is really really nice, and is so unbelievably busy yet he's managed to make time to help Kate out with the music -- the processional music -- and stuff like that."
"He's obsessed with his music," said Harry. "So the two of them have been slaving away, getting everything right. So it's fantastic really."
Kate seems like she was born for royalty.
"She gets on terribly well with Charles and Camilla," says royal expert Katie Nicholl. "Things like that are very important. She's very at ease when she's at the royal palaces. All of the things, I think, that Diana wasn't."