Early research in Amazonia suggested the possibility that prehistoric complex societies had developed in several regions, despite assumptions that humid tropical conditions would prevent such developments. Under the rubric of cultural ecology, various processes have been hypothesized for the development of these societies: invasion and subsequent devolution of groups from expanding states in temperate regions outside Amazonia, social and ecological interactions among regions within Amazonia, and social adaptation to local ecological variation. The hypotheses differ, but researchers generally employed ecological determinist and functionalist assumptions of causality: from environment to subsistence and population and thence to social adaptation. Recent thinking on complex society has distilled the concept of heterarchy as an alternative to cultural materialist explanations for the processes of formation and functioning of a range of complex societies. This chapter examines the accumulated data on complex societies in two Amazonian regions—Marajo Island at the mouth of the Amazon and the Santarem-Monte Alegre region in the Lower Amazon—in light of the theoretical issues about the formation and functioning of complex societies worldwide. Results of the comparison tend to accord more with heterarchical hypotheses than with the earlier cultural ecological hypotheses. In Amazonia, non-state societies appear to have organized large, dense populations, intensive subsistence adaptations, large systems of earthworks, production of elaborate artworks and architecture for considerable periods of time. The more centralized and hierarchical of these societies had developed more ritual and material culture related to conflict, and had a heavier impact on their environments. The patterns of social development in Amazonia can still be causally related to environmental patterns through cultural ecological theory, but the new data suggests the need to envision a more mutualistic, variable, and complex causal nexus.