From October to November of 1803, Lewis and Clark, while traveling down the Ohio River, began hiring the men who would join them on the journey west. News of the expedition had traveled fast, and there were many young men eager to be a part of it. Not only did the job promise excitement and adventure, the government also promised fine pay in both cash and land. Since this was officially a military venture, those men who were not already in the army would have to be sworn into it. Together, the men who comprised the permanent party that traveled the entire journey became known as the "Corps of Discovery."

The men who were hired to join Lewis and Clark were generally excellent frontiersmen. Lewis and Clark looked for men with a variety of skills, hiring some as interpreters, some for their skills as rivermen, and some who were hunters and trappers. All needed to be able to endure years of hard work and travel. Some of the men were of mixed ancestry, particularly of Native American and European descent.

Many of the men who traveled with the Lewis and Clark expedition went on to successful careers. One who led an especially interesting life after the expedition was John Colter. On August 14, 1806, nearing the end of the expedition, Colter was granted special permission to leave the Corps. He headed back out west, to become one of the first American mountain men and one of the first white men to enter and describe what is now Yellowstone National Park.