All complex and large-scale societies are organized along certain hierarchies, but the concept of hierarchy has become almost taboo in the modern world. This book contends that this stigma is a mistake. In fact, as the book shows, it is neither possible nor advisable to do away with social hierarchies. The book ask which forms of hierarchy are justified and how these can serve morally desirable goals. It looks at ways of promoting just forms of hierarchy while minimizing the influence of unjust ones, such as those based on race, sex, or caste. Which hierarchical relations are morally justified and why? The book argues that it depends on the nature of the social relation and context. Different hierarchical principles ought to govern different kinds of social relations: what justifies hierarchy among intimates is different from what justifies hierarchy among citizens, countries, humans and animals, and humans and intelligent machines. Morally justified hierarchies can and should govern different spheres of our social lives, though these will be very different from the unjust hierarchies that have governed us in the past. The book examines how hierarchical social relations can have a useful purpose, not only in personal domains but also in larger political realms.