 We begin by identifying four core features that distinguish justice from other moral and political ideas. We then examine some major conceptual contrasts: between conservative and ideal justice, between corrective and distributive justice, between procedural and substantive justice, and between comparative and non-comparative justice. Next we turn to questions of scope: to who or what do principles of justice apply? We ask whether non-human animals can be subjects of justice, whether justice applies only between people who already stand in a particular kind of relationship to one another, and whether individual people continue to have duties of justice once justice-based institutions have been created. We then examine three overarching theories that might serve to unify the different forms of justice: utilitarianism, contractarianism, and egalitarianism. But it seems, in conclusion, that no such theory is likely be successful.