With a focus on Cicero’s conception of justice and its lasting intellectual legacy, the JustCity project delves into one of the most innovative and influential, yet widely neglected contributions the history of Western political thought has to offer. Not only does Cicero’s law-centred conception of justice, both within and between states, mark a significant departure from the virtue-centred conceptions of justice typical for earlier, Greek theories, it also paved the way for what has come to be known as the constitutionalist tradition in political thought, as well as for the emergence of natural and international law in early modern Europe. In order to fully appreciate its massive impact in the long term, the project will trace Cicero’s conception of justice as it is first and foremost expressed in the famous Carneadean debate in the Republic through four historical inflection points: (i) its inception in the late Roman Republic; (ii) its transmission by the Christian writers Lactantius and Augustinus; (iii) its use by Alberico Gentili and other early modern thinkers engaged in debates on international politics and the law of nations; (iv) its effect on 18th century Enlightenment thought, as mediated by early 17th century natural law theorists. In connecting these four inflection points, which designate the four main themes of the project, JustCity pursues a longue durée intellectual history of one of the most fundamental and controversial concepts of Western political thought. The project is methodologically innovative and shows how the reception of classical antiquity can be operationalized and made fruitful for a long-term perspective in the history of ideas.