The notion of 'equity' is undergoing conceptual repositioning in international law today, embracing individuals as well as states and gaining an association with human rights and the politics of protest. In the context of these developments, the present paper enquires into the premodern roots of this ancient and rich term through three historical vignettes: first, the emergence of aequitas in Roman law -as a source of law anchored in analogy and empathy -and in particular its relevance to the ambiguous status of slaves; second, the importance of 'natural equity' to the consolidation of 'natural rights' during the Franciscan poverty debate in 14th century Europe, and finally, 'common equity' in the rights-based constitutional order proposed by the Levellers in 1640s England. In its root sense, I conclude, what we might call 'radical equity' has historically lent itself to trenchant critique of the law, centred on the individual as subject of right.Something interesting is happening to equity. Traditionally, in international law, 'equity' has referred to relations between countries, notably regarding land and sea boundaries and resource access. But recently the term has come to refer also to the treatment of individuals. This trend is visible in international climate law -where a 'principle' of equity initially referred, in 1992, not only to inter-state relations but also to 'present and future generations of humankind', and the 2015 Paris Agreement later extolling 'intergenerational equity' as well as 'equitable access' to 'sustainable development and eradication of poverty'. 1 But a similar trend has long been apparent at the World Health Organisation, where equity has been a key term of art since at least the 1980s, concerned with 'systematic differences in health status between different socioeconomic groups', 2 and also in more recent international proclamations such as the 2015 Sustainable Development Goals, which speak of 'equitable access' to education, drinking water, sanitation and hygiene for 'all people'. Beyond this again, 'equity' has come to inform global-level protests targeting inequity: ex- * Associate Professor, School of Law, London School of Economics and Political Science. My thanks to Martin Loughlin, Susan Marks, and the anonymous reviewers for helpful, and in some cases indispensable, comments. Any mistakes are my own.