Stone circles known as tipi rings are a common feature of the Northern and Northwestern Plains of western North America. There are several types of stone circles: (1) a single course of stone in a simple circle; (2) more complex circles with several courses of stones; (3) circular walls made of piled-up stones; (4) corrals or forts of horizontal logs and stones; (5) stone circles of unusual size or with lines of rocks inside the circle. Studies of tipi-ring sites in Montana, Wyoming, and adjacent regions are summarized. The clusters of circles are usually found along main routes of travel and near water, fuel, and good hunting areas. Although many of these sites have been mapped, only a few of them have been excavated. Despite the paucity of cultural remains found in the excavations, the tipi-ring clusters are generally considered to be the archaeological remains of camps and villages of people who dwelt in conical, skin-covered lodges. The stones are believed to have been used to hold the skin lodge cover snug against the ground. Historical records, ethnographic observations, and the recollections of present-day Indian informants confirm these interpretations. Data from 136 recently studied tipi-ring sites along a 300-mile pipe-line from Greenriver, Wyoming, to Denver, Colorado, provide further archaeological demonstration of the domestic origin of the tipi rings. Only the very small or very large rings, or those with spokes or lines of rocks through them, such as the so-called medicine wheels, are considered to have served a ceremonial purpose.