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. After a rainy winter, the Corps of Discovery turned homeward in March 1806 from Fort Clatsop on the mouth of the Columbia River. Detained by winter snows, they camped among the friendly Nez Perces in modern west-central Idaho. Captains Lewis and Clark attended to sick Indians and continued their scientific observations while others in the party hunted and socialized with Native peoples. 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.