In recent years a growing number of states have chosen to recognise environmental issues in their national constitutions. Some have added declarations about the value of the environment, some have sought to restrict or regulate government’s ability to take action which would potentially harm the environment, while others have proclaimed that citizens possess a right to an environment of a particular quality. A survey of these constitutional provisions reveals that the majority of reform in this area has come from developing states, including a number of states which have been designated as among the least developed countries in the world. The increasing focus on constitutional environmental rights appears to represent a shift in the attitude of developing and emerging economies, which could in turn be influential in setting the tone of the environmental rights debate more broadly, with potential to shape the future development of international law in the area. This chapter examines constitutional environmental rights in an attempt to determine whether consistent state practice can in fact be identified in this area which might form the basis of an emerging norm. It will also analyse some of the potential contributing factors to the proliferation of a constitutional right to a good environment among developing states, and the implications for the development of customary international law.