That's not how Mary-Kate and Ashley do it. "We don't really do licensing," explains Thorne. "Because licensing means you're giving rights to someone else." Instead, theirs is a do-it-yourself operation. Their company, Dualstar, employs eight full-time designers, a four-person art department, and 14 publicists in several countries. Together they turn out designs for clothing and other goods, and then Dualstar hands over the designs to manufacturers, mostly in the Far East, that make the products under contract.

Controlling the design and manufacture in this manner - so different from what most celebrities do - lets Dualstar ensure that the girls don't lend their fame to excessively cheap or shoddy products. The company also unquestionably adds value with their designs. "The Mary-Kate and Ashley team, headed up by Mary-Kate and Ashley themselves, has very strong designers," says Celia Clancy, senior vice president for ladies' and girls' apparel at Wal-Mart, the exclusive U.S. distributor of their clothing. "Mary-Kate and Ashley bring a new level of fashion to the girlswear assortment at Wal-Mart."

For its part, Wal-Mart has bought a lot of Mary-Kate and Ashley. In fact, an exclusive deal inked with the leading retailer in 2001 may well be more important than any amount of twin cuteness or brand equity built on video. "The primary driver [of Dualstar's success] has been its relationship with Wal-Mart," says licensing chronicler Brochstein.

Mary-Kate and Ashley have certainly brought a whole new level of attention to U.S. 'tweens - the roughly 27 million 6- to 12-year-olds who control some $250 million in annual spending. The highly visible success of Mary-Kate and Ashley has spurred imitators like Hilary Duff, who also made a name on television before launching a CD and, now, apparel and other branded products distributed through Target stores. So far, no other child sensation has made a dent in the Olsen juggernaut, but just as trees don't grow to the sky, surely something will happen to slow their growth.