 The idea of justice occupies centre stage both in ethics, and in legal and political philosophy. We apply it to individual actions, to laws, and to public policies, and we think in each case that if they are unjust this is a strong, maybe even conclusive, reason to reject them. Classically, justice was counted as one of the four cardinal virtues (and sometimes as the most important of the four); in modern times John Rawls famously described it as ‘the first virtue of social institutions’ (Rawls 1971, p.3; Rawls, 1999, p.3). We might debate which of these realms of practical philosophy has first claim on justice: is it first and foremost a property of the law, for example, and only derivatively a property of individuals and other institutions? But it is probably more enlightening to accept that the idea has over time sunk deep roots in each of these domains, and to try to make sense of such a wide-ranging concept by identifying elements that are present whenever justice is invoked, but also examining the different forms it takes in various practical contexts. This article aims to provide a general map of the ways in which justice has been understood by philosophers, past and present.