In 1803, when the United States purchased Louisiana from France, the great expanse of this new American territory was a blank—not only on the map but in our knowledge. President Thomas Jefferson keenly understood that the course of the nation’s destiny lay westward and that a national “Voyage of Discovery” must be mounted to determine the nature and accessibility of the frontier. He commissioned his young secretary, Meriwether Lewis, to lead an intelligence-gathering expedition from the Missouri River to the northern Pacific coast and back. From 1804 to 1806, Lewis, accompanied by co-captain William Clark, the Shoshone guide Sacajawea, and thirty-two men, made the first trek across the Louisiana Purchase, mapping the rivers as he went, tracing the principal waterways to the sea, and establishing the American claim to the territories of Idaho, Washington, and Oregon. Together the captains kept a journal, a richly detailed record of the flora and fauna they sighted, the Indian tribes they encountered, and the awe-inspiring landscape they traversed, from their base camp near present-day St. Louis to the mouth of the Columbia River. In keeping this record they made an incomparable contribution to the literature of exploration and the writing of natural history.