A pioneer of the medium, Timothy O’Sullivan was among the first people to call himself a photographer, taking photography out of the portrait studio and into the field, and ultimately creating some of the most iconic images of the American Civil War and the rugged Western frontier. O’Sullivan began his career working for Matthew Brady and then Alexander Gardner, who included forty-four of O’Sullivan’s battlefield photographs in Gardner’s Photographic Sketch Book of the War, the first published collection of Civil War photographs. After the war, Timothy O’Sullivan served as the official photographer for three ambitious government survey expeditions: the Geological Exploration of the Fortieth Parallel (or “King Survey”) of 1867–69 and 1872; the Darien (Isthmus of Panama) Expedition of 1870; and the Geological Surveys West of the 100th Meridian (or “Wheeler Survey”) of 1871 and 1873–74. The body of work he created during these expeditions is without precedent in its melding of scientific investigation with visual and emotional complexity. Working alongside geologists and surveyors, O’Sullivan produced some of the earliest and most enduring photographs of the American frontier. Soon after being appointed chief photographer for the United States Treasury in 1880, O’Sullivan died of tuberculosis at age forty-one.