Few individuals have exerted as strong an influence on twentieth-century American art and culture as the photographer and art dealer Alfred Stieglitz. Born in Hoboken, New Jersey, in 1864 during the Civil War, Stieglitz lived until 1946. He began to photograph while a student in Berlin in the 1880s and studied with the renowned photochemist Hermann Wilhelm Vogel. On his return to the United States in 1890, he began to advocate that photography should be treated as an art. He wrote many articles arguing his cause, edited the periodicals Camera Notes (1897-1902) and Camera Work (1903-1917), and in 1902 formed the Photo-Secession, an organization of photographers committed to establishing the artistic merit of photography. Stieglitz photographed New York for more than twenty-five years, portraying its streets, parks, and newly emerging skyscrapers; its horse-drawn carriages, trolleys, trains, and ferry boats; as well as some of its people. In the late 1910s and early 1920s, he also focused his camera on the landscape around his summer home in Lake George, New York. In 1918 Stieglitz became consumed with photographing his future wife, the artist Georgia O'Keeffe. For many years he had wanted to make an extended photographic portrait--he called it a composite portrait--in which he would study one person over a long period. Over the next nineteen years he made more than 330 finished portraits of her. Beginning in 1922 and continuing throughout the 1920s, he also became preoccupied with another subject, clouds, making more than 300 finished studies of them. Stieglitz witnessed some of the most profound changes this country has ever experienced: two world wars, the Great Depression, and the growth of America from a rural, agricultural nation to an industrialized and cultural superpower. But, more significantly, he also helped to affect some of these transformations. Through his New York galleries--the Little Galleries of the Photo-Secession at 291 Fifth Avenue, which he directed from 1905 to 1917; The Intimate Gallery, 1925-1929; and An American Place, 1929-1946--he introduced modern European art to this country, organizing the first exhibitions in America of work by Pablo Picasso, Henri Matisse, Georges Braque, and Paul Cézanne, among others. In addition, he was one of the first to champion and support American modernist artists such as Georgia O'Keeffe, Arthur Dove, John Marin, Marsden Hartley, and Charles Demuth. Photography was always of central importance to Stieglitz: not only was it the medium he employed to express himself, but, more fundamentally, it was the touchstone he used to evaluate all art. Just as it is apparent today that computers and digital technology will dominate not only our lives but also our thinking in this century, so too did Stieglitz realize, long before many of his contemporaries, that photography would be a major cultural force in the twentieth century. Fascinated with what he called "the idea of photography," Stieglitz foresaw that it would revolutionize all aspects of the way we learn and communicate and that it would profoundly alter all of the arts. Stieglitz's own photographs were central to his understanding of the medium: they were the instruments he used to plumb both its expressive potential and its relationship to the other arts. When he began to photograph in the early 1880s, the medium was barely forty years old. Complicated and cumbersome and employed primarily by professionals, photography was seen by most as an objective tool and utilized for its descriptive and recording capabilities. By the time ill health forced Stieglitz to stop photographing in 1937, photography and the public's perception of it had changed dramatically, thanks in large part to his efforts. Through the publications he edited, including Camera Notes, Camera Work, and 291; through the exhibitions he organized; and through his own lucid and insightful photographs, Stieglitz had conclusively demonstrated the expressive power of the medium.