Why do some countries adopt constitutional environmental rights while others do not? This article uses qualitative content analysis of interviews conducted in Kathmandu (Nepal) and Colombo (Sri Lanka) to analyze the cases of Nepal, which adopted a constitutional environmental right in the 2007 Interim Constitution, and Sri Lanka, which has not enacted such a right in any of its governing charters. It finds that the presence of a constitutional environmental right in Nepal and the absence of such a right in Sri Lanka can be best explained directly with reference to domestic political conditions and structures, and indirectly in terms of the international normative environment in which the constitution was written. The article outlines a research agenda which focuses on evaluating the impacts of constitutional environmental rights. This research provides important insights into the process of constitutional design in developing states and the translation of international norms in domestic contexts.