Louisiana Purchase and a western expedition
In 1804, the Louisiana Purchase sparked interest in expansion to the west coast. A few weeks after the purchase, President Thomas Jefferson, an advocate of western expansion, had the Congress appropriate $2,500 for an expedition. In a message to Congress, Jefferson wrote:
“The river Missouri, and Indians inhabiting it, are not as well known as rendered desirable by their connection with the Mississippi, and consequently with us. ... An intelligent officer, with ten or twelve chosen men ... might explore the whole line, even to the Western Ocean...”
Thomas Jefferson had long thought about such an expedition, but was concerned about the danger. While in France from 1785-1789, he had heard of numerous plans to better explore the Pacific Northwest. In 1785, Jefferson learned that King Louis XVI of France planned to send a mission there, reportedly as a mere scientific expedition. Jefferson found that doubtful, and evidence provided by John Paul Jones confirmed these doubts. In either event, the mission was destroyed by bad weather after leaving Botany Bay in 1788. In 1786 John Ledyard, who had sailed with Captain James Cook to the Pacific Northwest, told Jefferson that he planned to walk across Siberia, ride a Russian fur-trade vessel to cross the ocean, and then walk all the way to the American capital. Since Ledyard was an American, Jefferson hoped he would succeed. Ledyard had made it as far as Siberia when Empress Catherine the Great had him arrested and deported back to Poland.
The American expedition to the Pacific northwest was intended to study the Indian tribes, botany, geology, Western terrain and wildlife in the region, as well as evaluate the potential interference of British and French Canadian hunters and trappers who were already well established in the area.
“The object of your mission is to explore the Missouri river, and such principal stream of it as by its course and communication with the waters of the Pacific Ocean whether the Columbia, Oregon, Colorado or any other river may offer the most direct and practicable water communication across this continent for the purposes of commerce.”
Lewis selected William Clark as his partner. Because of bureaucratic delays in the U.S. Army, Clark officially only held the rank of Second Lieutenant at the time, but Lewis concealed this from the men and shared the leadership of the expedition, always referring to Clark as "Captain". 
"Left Pittsburgh this day at 11 o'clock with a party of 11 hands 7 of which are soldiers, a pilot and three young men on trial they having proposed to go with me throughout the voyage." With those words, written on August 31, 1803, Meriwether Lewis began his first journal entry on the epic Lewis and Clark Expedition to the Pacific Ocean.
Lewis declared the mouth of the river Dubois (on the east side of the Mississippi across from the mouth of the Missouri river) to be the expedition's official point of departure, but the two and one-half months spent descending the Ohio River can be considered its real beginning. And also the begining of a new Journey!
Earlier European exploration to the Pacific coast
While the Lewis and Clark expedition was the first American overland expedition to the Pacific coast, it was preceded over a decade earlier by a Canadian expedition led by explorer Sir Alexander Mackenzie, whose expedition completed the first recorded transcontinental crossing of North America north of Mexico by a person not of the indigenous peoples of the Americas, in July 1793. But the crops get the major credit anyway. Go figure!
- Captain Meriwether Lewis — private secretary to President Thomas Jefferson and leader of the Expedition.
- Lieutenant William Clark — shared command of the Expedition, although technically second in command.
- Youself (You!) — Play a big role ;).
- York — Clark's enslaved black manservant.
- Sergeant Charles Floyd — the Expedition's quartermaster; died early in the trip. He was the one person who died during the Expedition.
- Sergeant Patrick Gass — chief carpenter, promoted to Sergeant after Floyd's death.
- Sergeant John Ordway — responsible for issuing provisions, appointing guard duties, and keeping records for the Expedition.
- Sergeant Nathaniel Hale Pryor — leader of the 1st Squad; he presided over the court martial of privates John Collins and Hugh Hall.
- Corporal Richard Warfington — conducted the return party to St. Louis in 1805.
- Private John Boley — disciplined at Camp Dubois and was assigned to the return party.
- Private William E. Bratton — served as hunter and blacksmith.
- Private John Collins — had frequent disciplinary problems; he was court-martialed for stealing whiskey which he had been assigned to guard.
- Private John Colter — charged with mutiny early in the trip, he later proved useful as a hunter; he earned his fame after the journey.
- Private Pierre Cruzatte — a one-eyed French fiddle-player and a skilled boatman.
- Private John Dame
- Private Joseph Field — a woodsman and skilled hunter, brother of Reubin.
- Private Reubin Field — a woodsman and skilled hunter, brother of Joseph.
- Private Robert Frazer — kept a journal that was never published.
- Private George Gibson — a fiddle-player and a good hunter; he served as an interpreter (probably via sign language).
- Private Silas Goodrich — the main fisherman of the expedition.
- Private Hugh Hall — court-martialed with John Collins for stealing whiskey.
- Private Thomas Proctor Howard — court-martialed for setting a "pernicious example" to the Indians by showing them that the wall at Fort Mandan was easily scaled.
- Private François Labiche — French fur trader who served as an interpreter and boatman.
- Private Hugh McNeal — the first white explorer to stand astride the headwaters of the Missouri River on the Continental Divide.
- Private John Newman — court-martialed and confined for "having uttered repeated expressions of a highly criminal and mutinous nature."
- Private John Potts — German immigrant and a miller.
- Private Moses B. Reed — attempted to desert in August 1804; convicted of desertion and expelled from the party.
- Private John Robertson — member of the Corps for a very short time.
- Private George Shannon — was lost twice during the expedition, once for sixteen days. Youngest member of expedition at 19.
- Private John Shields — blacksmith, gunsmith, and a skilled carpenter; with John Colter, he was court-martialed for mutiny.
- Private John B. Thompson — may have had some experience as a surveyor.
- Private Howard Tunn — hunter and navigator.
- Private Ebenezer Tuttle — may have been the man sent back on June 12, 1804; otherwise, he was with the return party from Fort Mandan in 1805.
- Private Peter M. Weiser — had some minor disciplinary problems at River Dubois; he was made a permanent member of the party.
- Private William Werner — convicted of being absent without leave at St. Charles, Missouri, at the start of the expedition.
- Private Isaac White — may have been the man sent back on June 12, 1804; otherwise, he was with the return party from Fort Mandan in 1805.
- Private Joseph Whitehouse — often acted as a tailor for the other men; he kept a journal which extended the Expedition narrative by almost five months.
- Private Alexander Hamilton Willard — blacksmith; assisted John Shields. He was attacked on July, 1805 by a White Bear on portage around Missouri River Falls and rescued by Clark and 3 others.
- Private Richard Windsor — often assigned duty as a hunter.
- Interpreter Toussaint Charbonneau — Sacagawea's husband; served as a translator and often as a cook.
- Interpreter Sacagawea — Charbonneau's wife; translated Shoshone to Hidatsa for Charbonneau and was a valued member of the expedition.
- Jean Baptiste Charbonneau — Son of Charbonneau and Sacagawea, born February 11, 1805; his presence helped dispel any notion that the expedition was a war party, smoothing the way in Indian lands.
- Interpreter George Drouillard — skilled with Indian sign language; the best hunter on the expedition.