There’s no question that Catherine, Duchess of Cambridge has enviable skin—and we tracked down how she gets it! According to New York celebrity dermatologist David Colbert, his longtime client Sienna Miller passed along a royal referral. “Kate saw my products in Sienna’s dressing room when she paid her a visit backstage after Sienna’s play [Flare Path] this year,” Colbert tells us. “Now clients are asking for ‘The Princess Serum.’ We’ve been selling out everywhere. I call it the Kate Effect.” While Colbert has yet to see Middleton for an in-office appointment, he did give InStyle.com the very products he’d recommend to her. Click “See the Photos” to check them out.