In 1949 Georgia O'Keeffe chose the National Gallery of Art as the custodian of nearly 1,600 photographs by Alfred Stieglitz - the Key Set, as it has become known. With the formation in 1990 of the Gallery's department of photographs under Sarah Greenough, the collection has grown to 14,000 works of art, an assemblage that both charts the development of the medium and reveals the beauty and dynamic versatility of photography over its course of more than 175 years. This elegant book presents some of the most significant and compelling photographs acquired over the years, ranging from experimental photographs made in the earliest years of the medium's history to key works by major twentieth-century figures and contemporary pieces that reset the ways in which photography shapes our experience of the modern world. The guides on this enlightening walk through the history of the medium are members of the extraordinary curatorial team that established the National Gallery's international reputation for photography exhibitions and publications over the past twenty-five years, ever advancing the recognition of photography as a fine art.