AbstractSocial sustainability plays a prominent role in the United Nation’s Sustainable Development Goals, but a proper analysis of the concept is still lacking. According to a widespread conception, a system is sustainable when it is preserved or developed in a robust manner. I argue, however, that social sustainability is best understood in explicitly normative terms. Formulating suitable development goals requires a conception of the kind of society that is worth sustaining. I propose that, for a system to be socially sustainable is for it to secure a range of values, including justice. Furthermore, I argue that social sustainability is first and foremost a property of institutions. I go on to ask what it takes for an institution to secure values and why justice is so important for institutions. The answers culminate in a social sustainability framework, which explains why making institutions more sustainable is so challenging.