On the one hand, human behavior and its determinants can be seen in terms of a relatively simple “input-output” system. On the other hand, it is also possible to envisage a more complex interplay between behavior and its determinants unfolding at multiple environmental levels. A key premise of this chapter is that planned behavior change programs should target not only the individual but also the environmental influences on behavior at the interpersonal, organizational, community, and societal levels. Each environmental level encompasses physical, social, and cultural dimensions. Two key ecological assumptions help us to identify intervention targets for promoting behavior change. First, behavior influences, and is influenced by, multilevel environmental factors; second, individual behavior both shapes and is shaped by the environment. The socioecological approach and the accompanying range of theoretical approaches described in this chapter do justice to both perspectives. This approach enables researchers to apply insights from theoretical frameworks at the individual, interpersonal, organizational, community, and societal levels. The resulting multilevel interventions can target complex phenomena such as power differences, social networks, diffusion of innovations, organizational change, coalition building, and policy processes.