Arapahos referred to themselves as Hinanaeina (hinono'eino), or "The People." Trappers and traders in the early nineteenth century used the Crow name for Arapahos, Alappaho (Many Tattoos), and Arapahos began referring to themselves by that term in their dealings with Americans. Five dialects of Arapaho (an Algonquian language) existed in historical times and correlated with tribal divisions: Hinanaeina (Arapaho); Hitounena (Gros Ventre); and three others, the speakers of which presumably became absorbed by the other divisions.

Arapahos entered the Northern Plains at least by the early eighteenth century, probably from the northeast. The Gros Ventre division remained in the far Northern Plains, while Arapahos moved in a southerly direction. Wealthy in horses, probably since the 1740s, Arapahos ranged from the headwaters of the Missouri to the Platte River, and west as far as the foothills of the Rocky Mountains. By 1806 they allied with Cheyennes, largely to counter the westward movement of the Sioux (Lakota). With the Cheyennes, they drove the Kiowas and Comanches south of the Arkansas River by 1826 and controlled the region between the Platte and Arkansas Rivers. In 1835, diminished by smallpox, they numbered an estimated 3,600.

Arapahos relied on bison for food, clothing, and many other necessities. In the 1830s and 1840s they hunted in the Estes Park area of Colorado (especially the region of the Cache la Poudre River) and the adjacent Plains to the east, which they recognized as their exclusive territory. Men hunted and women dried the meat, collected and dried roots and berries, dressed hides, and made tipi covers, clothing, and containers. The quilled, painted, and, by the nineteenth century, beaded designs applied on hide by women represented prayers for the well being of a relative. After 1857 settlers and miners moved into the Parks area of presentday Colorado, driving off the bison, so that the Arapahos had to hunt more regularly on the open plains east of the Rockies.

Most of the year, Arapahos lived in bands that moved together when large camps were formed or disbanded and affiliated with one of the several named subdivisions of the Hinanaeina, but individuals and households could move from one band to another. Kinship was reckoned on the basis of bilateral descent, although other individuals could be absorbed into kindred by means of "adoption." Arapahos married outside their group of kindred and legalized marriage by gift exchange between the bride's and groom's families.

The Arapaho origin story focuses on Pipe Person's creation of the earth from mud below the surface of an expanse of water. Pipe Person, through prayer-thought, created all life, including the first Arapahos. Arapahos henceforth kept a replica of the Flat Pipe as a symbol of their covenant with the life force or power on which Pipe Person drew. Rites centered on the pipe bundle helped ensure the success of Arapahos generally and of individuals specifically. Seven men's and seven women's medicine bags contained objects and implements that symbolized forms of power, and these passed from one custodian to another. Prayerthoughts could affect events and lives, and the sincerity of a petitioner's prayer-thought was validated by sacrifices of property or of the body by flesh offerings and fasting. In the major tribal ritual, the Offerings Lodge (also known as the Sun Dance), a petitioner vowed to participate (that is, make a sacrifice) in the ceremony in return for supernatural aid. Individuals also acquired supernatural aid by dreaming or fasting for a vision encounter with a supernatural being. During the early nineteenth century, many of the men's vision fasts were on various peaks in Estes Park, Colorado. Women usually received power in a dream or from a husband or parent.

A governing council and an age-graded series of societies, supervised by the elderly custodians of the medicine bundle, comprised the tribal government. Initiation into each society was precipitated by a religious vow; the age mates of the votary went through the ceremony as a group. Wives were considered to progress through the men's societies with their husbands. These men's groups performed political duties, including keeping order in the camp and supervising the communal hunts. The governing council in the nineteenth century consisted of four leaders representing four tribal subdivisions, the medicine bundle custodians, and the leaders of the men's societies. Beginning in the 1840s "chiefs" served as intermediaries between the governing council and federal officials.

The Arapahos prospered from trading bison robes to Americans in the 1830s, but beginning in the 1840s, American expansion westward disturbed the bison herds. The United States initiated treaty councils, first to prevent troubles along the immigrant routes and later to remove Plains peoples from areas where Americans wanted to settle. In 1851 the Arapahos signed a peace treaty that guaranteed that settlers would not trespass on the tribe's lands in Wyoming and Colorado. Settlers and miners violated the treaty, with little opposition from the federal government, which led to trouble with the Arapahos and Cheyennes. In 1864 Colorado militia massacred a Cheyenne and Arapaho camp at Sand Creek, provoking a two-year war and resulting in the separation of Arapaho bands into politically independent northern and southern divisions. In 1867 the Southern Arapahos, led by Little Raven, signed the Treaty of Medicine Lodge Creek, ending hostilities. In 1868 the Northern Arapahos, having fought for several years to hold onto the bison range in Wyoming and Montana, signed a treaty under Medicine Man's leadership and agreed to settle on a reservation. Intermediary chiefs negotiated with officials during subsequent months, and in 1869 President Ulysses S. Grant created by executive order a reservation for Southern Arapahos and Cheyennes in Indian Territory (later Oklahoma). In 1878 Northern Arapahos obtained permission to settle on the Shoshone reservation in Wyoming.

When they moved to their reservations, the Southern Arapahos numbered about 1,200 and the Northern Arapahos, 1,000. The Southern branch continued to hunt bison until 1878, when game became scarce. During the 1880s Arapahos on both reservations depended for subsistence on the supplies issued by the federal government and disbursed by band leaders, the most important of whom were Black Coal and Sharp Nose in Wyoming and Powderface, Left Hand, and Yellow Bear in Oklahoma. Leaders also organized communal agricultural labor on both reservations and freighting and livestock raising in Oklahoma. The reservation in Oklahoma was divided into individually owned allotments of land in 1892, and unallotted lands were sold to non-Indians. In Wyoming, allotment occurred in 1901, but the unallotted lands were in an area undesirable for farming, and consequently these lands were never sold. In Oklahoma, the federal government facilitated the sale of most of the allotments over the years, while in Wyoming oil was discovered on the tribally owned unallotted lands. Since 1940 all Northern Arapahos have received monthly per capita payments from mineral royalties and bonuses, which have helped to alleviate poverty and have kept land sales to a minimum.

Both the Oklahoma and Wyoming Arapahos had intermediary chiefs until the third decade of the twentieth century. These chiefs worked to defend the treaty rights of their respective tribes. When the federal government encouraged the formation of elective, representative, and constitutional government, Oklahoma Arapahos instituted a "business committee." Wyoming Arapahos rejected the idea of constitutional government, but adopted an elective, representational "business council." In both cases, traditional ideals of leadership became incorporated into the new form of tribal government. Federal programs that created jobs, scholarships, housing, and other kinds of development were introduced, beginning in the 1960s, and these strengthened the role of tribal government. In the late 1990s the population of Arapahos in Oklahoma was about 4,000, in Wyoming at that time there were about 4,400 Arapahos̵half of whom live within the boundaries or former boundaries of the reservation. Enrollment in the Northern or Southern Arapaho tribes is contingent on having at least 25 percent Arapaho ancestry; most have more than 25 percent.

The O'erings Lodge continued in Oklahoma until 1939, and thereafter Arapahos took it to Wyoming, where the ceremony continues today, as does the Sacred Pipe ritual. The Ghost Dance movement was important in the 1890s and, in revised form, in the early twentieth century in both the Oklahoma and Wyoming communities. Peyote ritual was introduced to the Southern Arapahos by Plains Apaches in the 1890s and then transferred to the Wyoming Arapahos; the Native American Church continues to be important in both communities. Mennonite and Baptist missionaries introduced Christianity in Oklahoma, and Catholic and Episcopal missionaries in Wyoming. Generally, the practice of Native religion and Christianity are not mutually exclusive. Since the 1970s there has been an elaboration of "traditional" religious and social ritual in both the Oklahoma and Wyoming communities.

See also WAR: Sand Creek Massacre.