In 1800, Spain ceded a vast area, including Colorado, to Napoleon Bonaparte and the French. Three years later, the same parcel of land was sold by Napoleon the United States as the "Louisiana Purchase". In 1806, President Jefferson commissioned Lieutenant Zebulon Pike to explore the recently purchased territory. Among the sites mentioned by Pike in his report of the expedition was the 14,110-foot peak, which today bears his name. Pike stated in his report that it was unlikely the summit would ever be scaled.

A group of explorers led by Major Stephen H. Long proved Pike to be wrong in 1820 when Dr. Edwin James and two others in the Long party became the first to climb to the summit of Pike's Peak. In making their journey, Long and his party passed the present day locations of Greeley, Denver, and Colorado Springs. They also viewed the mountain later known as Lounge's Peak.

At about the same time as the Long expedition, fur trappers and traders began working their trade in the Rocky Mountains. Beaver fur was considered on of the most precious commodities on the continent. The beaver pelt, small and light in weight, provide d a high-priced product in a small package. A single pelt sold in eastern markets for six to eight dollars. During the decade of the 1830's both the supply and price of beaver skins declined, forcing traders to turn to hunting buffalo. In addition, trading ding posts were established for barter with the Indians. Fort Pueblo (1842), Fort Vesquez (1835), and Bent Fort (1834) were the most important posts.

Many Indian tribes roamed Colorado and contributed to the state's history. The Cheyenne, Arapaho, Comanche, and Kiowa were the most important plains tribes. They were nomadic, hunting and making clothes from the skins of buffalo and deer, living in teepees, and depending on berries and roots for vegetables. The Spanish found Navajo in southwestern Colorado. The Apache frequently came into the state from New Mexico and Arizona. The Utes inhabited the state' s mountains and appear to have been the only indigenous tribe of Colorado. Utes remaining in Colorado today live in the southwestern corner of the state. The Cheyenne and Arapaho roamed the state's eastern plains.

The discovery of gold in California in 1849 touched off a search for gold in other regions including the Rocky Mountains and accounted for the first extensive settlement of Colorado. In July of 1858, William Green Russell, a Georgia miner, discovered several hundred dollars worth of gold at the mouth of Dry Creek in the present-day Denver suburb of Englewood. Russell's find started the "Pike's Peak or Bust" gold rush of 1858-59. Historians estimate that approximately 50,000 people came to Colorado in search of gold in 1858-59.

After Russell and his brothers made another gold discovery on Cherry Creek, General William Larimer led a group of men from the Kansas Territory to establish a settlement there. The resulting settlement was christened Denver City in honor of James W. Denver, governor of Kansas Territory. Cherry Creek provided a boundary between Denver City and another community established earlier, Auraria. Despite an initial rivalry, these two communities were consolidated into the single community of Denver in 1860. Gold deposits found in other areas led to the establishment of more towns. In particular, the discovery of gold forty miles west of Cherry Creek led to the establishment of the twin towns of Central City and Blackhawk. The first permanent white settlements in the state were in the San Luis Valley. The town of San Luis founded in 1851 is generally considered the oldest continually occupied town in Colorado.

In January of 1861, Congress voted statehood for Kansas. A bill to create Colorado Territory was passed almost immediately thereafter. President Lincoln appointed William Gilpin as the state's first territorial governor. The population of Colorado in 1861 was 21,000. The first legislature, sitting in Denver, selected Colorado City (west of present day Colorado Springs) as the capitol. The second legislature met there only a few days, in 1862, and adjourned to Denver. The assembly met in Denver and Golden up to 1867 when Denver was named the permanent seat of the territory. In the years following the establishment of the territory, numerous attempts were made to gain statehood for Colorado. However, it was not until 1876 - fifteen years after becoming a territory - that Colorado was admitted as the thirty-eighth state in the union. Colorado was called the "Centennial State" in honor of the one-hundredth year of the Declaration of Independence.