945 Madison Avenue, also known as the Breuer Building and formerly the Whitney Museum Building, is located on the Upper East Side of Manhattan, New York City. Built from 1964 to 1966 as the third home of the Whitney Museum of American Art, it subsequently held a branch of the Metropolitan Museum of Art and the Frick Collection before becoming the headquarters of Sotheby's auction house. Marcel Breuer and Hamilton P. Smith were the primary architects, with Michael H. Irving as the consulting architect and Paul Weidlinger as the structural engineer.