Sustainability, as compared to the rule of law, human rights, sovereignty or democracy, is a relatively new constitutional key concept. It is mentioned explicitly more and more in constitutional discourses, and – even more importantly – it helps to reconstruct a number of current constitutional debates under one conceptual umbrella. Sustainability comprises different responses to long-term social challenges which cannot efficiently be responded to via democratic mechanisms. Democratic mechanisms are based on election terms and are, consequently, structurally short-sighted. By ‘European constitutional law’, this chapter refers to both the primary law of the EU and domestic constitutional documents.