 Equality in its prescriptive usage is closely linked to morality and justice, and distributive justice in particular. Since antiquity equality has been considered a constitutive feature of justice. (On the history of the concept, cf. Albernethy 1959, Benn 1967, Brown 1988, Dann 1975, Thomson 1949.) People and movements throughout history have used the language of justice to contest inequalities. But what kind of role does equality play in a theory of justice? Philosophers have sought to clarify this by defending a variety of principles and conceptions of equality. This section introduces four such principles, ranging from the highly general and uncontroversial to the more specific and controversial. The next section reviews various conceptions of the ‘currency’ of equality. Different interpretations of the role of equality in a theory of justice emerge according to which of the four principles and metrics have been adopted. The first three principles of equality hold generally and primarily for all actions upon others and affecting others, and for their resulting circumstances. From the fourth principle onward, i.e., starting with the presumption of equality, the focus will be mainly on distributive justice and the evaluation of distribution.