Since the time of Columbus, explorers dreamed of a water passage across the North American continent. President Thomas Jefferson shared this dream. He conceived the Corps of Discovery to travel up the Missouri River to the Rocky Mountains and westward along possible river routes to the Pacific Ocean. Meriwether Lewis and William Clark led this expedition of 1804-6. Along the way they filled hundreds of notebook pages with observations of the geography, Indian tribes, and natural history of the trans-Mississippi West. This volume includes Lewis’s and Clark’s journals beginning in August 1803, when Lewis left Pittsburgh to join Clark farther down the Ohio River. The two men and several recruits camped near the mouth of the Missouri River for five months of training, acquiring supplies and equipment, and gaining information from travelers about the trip upriver. They started up the Missouri in May 1804. This volume ends in August, when the Corps of Discovery camped near the Vermillion River in present-day South Dakota. Gary E. Moulton is Thomas C. Sorensen Professor of American History at the University of Nebraska and the recipient of the J. Franklin Jameson Award of the American Historical Association for the editing of these journals.