The growing influence and self-confidence of local authorities count among the most interesting recent phenomena in global governance. While not entirely oblivious, international law as a field has struggled to get ahead of this dynamic, focusing instead on how to integrate local authorities into static conventional frameworks firmly based on the notion of state sovereignty. However, as a discussion of the global state of affairs and a focus on human rights cities shows, local actors increasingly claim and obtain a key role in the realization of international law. Additionally, they hold important potential to address some of the most pressing challenges to international human rights law concerning its efficacy and legitimacy. This article therefore calls for a proactive approach to the study of local authorities that considers local authorities as a ‘new frontier’ in international law generally and in human rights law specifically. It proposes a critical research agenda for this purpose that could produce important new insights into (i) the continued relevance and legitimacy of human rights as a discourse of governance; (ii) the bearing of domestic constitutional arrangements on the implementation of human rights law and (iii) questions of, and possible shifts in, legal subjecthood in the contexts of ‘state failure’.