As sustainability gains popularity in public discourse, scholars have noted its diverse uses, multiple meanings, and contradictory outcomes. This paper explores how the current proliferation of the concept of sustainability stems in part from its varied normative appeals, which in turn motivate, legitimate, and unsettle its diverse mobilizations. As the concept of sustainability calls for an extension of moral horizons beyond the immediate here and now, this redrawing of moral boundaries has simultaneously produced new externalities as well as enduring anxieties and responses within these moral bounds themselves. Drawing on ethnographic and historical materials, we argue that sustainability's moral boundaries have become both an object of scholarly critique and their own productive site of anxiety and negotiation. Questions about sustainability's moral horizons and externalities often surface in the concept's public deployment itself. We suggest that these tensions can be made visible by attending to the intersections between sustainability and a broader range of moral concerns at work.