The principle of sustainability is generally taken as a good, but what does sustainability really mean? The notion of sustainability has been at the center of global governance debates for more than a decade and many countries across the world include sustainability in their constitutions. This paper argues that in order to understand the concept of sustainability in a constitutional context, we need to turn to the notion of dignity. The paper explores the concepts of sustainability and dignity and their (...) meanings in the framework of climate justice and related questions and by discussing them in the context of Kantian and republican theory. In addition, the paper looks at intriguing court cases on the importance of nation states meeting their climate law duties. The paper concludes by exploring new uses of the sustainability concept. EU constitutional law will serve as a laboratory in testing these questions. (shrink)