The principle of equality and non-discrimination has gained a prominent status in virtually every liberal democratic state as well as in international law. However, what this fundamental rule entails in practice is difficult to establish. The challenge is to give substance to the abstract notion of equality by translating it into concrete legal formulations that clarify which forms of unequal treatment are legitimate because they are based on morally acceptable criteria and which ones are wrongful. This chapter explains how this challenge has been addressed in international human rights law. It first discusses the meaning of equality and non-discrimination and gives an overview of the different norms guaranteeing equality and non-discrimination in international human rights law, followed by an explanation of the concepts of direct and indirect discrimination. The chapter then considers the requirements for a difference in treatment to be justified under international human rights law and sets out the various obligations that the right to equality imposes on states, in particular their duty to take positive action to ensure everyone can enjoy that right.