The Plateau culture area of northwestern North America fits the criteria of an interaction sphere. Understanding the general cultural dynamics responsible for the creation of interaction spheres has been poorly developed in archaeological and ethnological theory. Data from the Plateau Interaction Sphere are used to argue that the main factor responsible for the emergence of interaction spheres in transegalitarian societies is the development of an elite class. Elites who seek to maximize their power and wealth at the tribal level do so in part by establishing trading, marriage, ideological, military, and other ties to elites in other communities and regions. They use these ties to monopolize access to desirable regional prestige goods and to enhance their own socioeconomic positions. In conformity with expectations derived from this model, the data from the Plateau demonstrate that interaction sphere goods are predominantly prestige items and that these concentrate in communities that have the greatest potential to produce surplus and to develop socioeconomic inequalities. These same features also seem to characterize well-known interaction spheres elsewhere in the world.