The Blackfoot Nation (Soyi-tapis, meaning "prairie people") is composed of the closely related the Blackfoot proper or Siksika, the Blood, and the Peigan (generally spelled "Piegan" in the US). All speak dialectic variants of the same language, which belongs within the Algonkian linguistic family. The English name for the Blackfoot is a direct translation of the word siksika, meaning "black foot". The Blood prefer to be called by the Blackfoot name, Kainai ("many chiefs"). The name "Peigan" is a corruption of the Blackfoot word apiku'ni, meaning "badly tanned robe".

Allied with the Blackfoot Nation historically were the Sarcee, Athapaskan- or Dene-speakers properly referred to as the Tsuu T'ina, and the Atsina (Gros Ventres), who speak an Algonkian language distantly related to Blackfoot. Historically, the three Blackfoot tribes, along with the Tsuu T'ina and Atsina, formed the Blackfoot Confederacy.

The Plains Cree, whose relations with the Blackfoot were frequently hostile during the 1800s, also speak an Algonkian language remotely related to Blackfoot. The term "Cree" is believed to be a contraction of Kiristinon, the name recorded in early French documents for a band of Native people who lived in the James Bay region. Most Cree people refer to themselves by particular band titles, using the generic English word, "Cree", only when communicating in English.

The people known in English as the Stoney, who separated from the Plains Assiniboine sometime before 1640 and moved westward with the Cree, speak Nakota, a northern dialect of the Dakota Sioux language. They refer to themselves as îyârhe Nakodabi or "Rocky Mountain Sioux". The term "Assiniboine" actually derives from an Ojibwa word describing the custom of boiling water by immersing hot stones in pits and skin containers filled with liquid. The practice, which was standard in the days before the introduction of metal pots, also accounts for the English title given to the Assiniboine bands of southern Alberta's mountains and foothills.