High Quality Content by WIKIPEDIA articles! The Lewis and Clark Expedition (1804 1806) was the first overland expedition undertaken by the United States to the Pacific coast and back. The expedition team was headed by Meriwether Lewis and William Clark and assisted by Sacajawea and Toussaint Charbonneau. The expedition's goal was to gain an accurate sense of the resources being exchanged in the Louisiana Purchase. The expedition laid much of the groundwork for the westward expansion of the United States.