Ecological modernization theory posits that even though economic development harms the environment, the magnitude of the harmful link decreases over the course of development. In contrast, the treadmill of production theory argues that the strong relationship between environmental harms and economic development will remain constant or possibly increase through time. To evaluate these competing propositions, interactions between economic development and time are used in cross-national panel analyses of three measures of carbon dioxide emissions. The results vary across the three outcomes as well as between developed and less developed countries, providing mixed support for both theoretical perspectives. The authors conclude by discussing how both theories could benefit from engaging contemporary research concerning changes within the transnational organization of production and the structure of international trade and how these global shifts influence environment/economic development relationships.