InStyle's fashion director, Hal Rubenstein waxes iconic in his new book, 100 Unforgettable Dresses (Harper Design, 2011). From Rita Hayworth in Gilda to Julia Roberts in Pretty Woman, Princess Diana to the Duchess of Cambridge, Grace Kelly to Grace Jones, Bob Mackie to Karl Lagerfeld, Rubenstein delves into fashion history and comes up with an informative and entertaining read. But what makes a dress unforgettable? Calvin Klein -- one of several designers interviewed for the book --  put it best: "an unforgettable moment is the combination of the right dress on the right woman at the right time. Click through for a sampling of some of the dresses that made the list.*

*Kate Middleton's Alexander McQueen was a late addition and Rubenstein had to cut one of the dresses to keep the list to 100. So it was sayonara Nancy Reagan and her white one-shoulder inauguration ball gown. Michelle Obama's Jason Wu, however, remained in tact. [WWD, sub req'd]