What is equality? Are there different kinds of equality? Who is equal? And what are the consequences of treating others equally? This work investigates what kinds of equality can be applied to animals in a consistent way. It starts at the meta-ethical level, how to construct a coherent ethical system of universal ethical principles. Counteracting arbitrariness is of key importance at this level. Next it descends to the level of normative ethics, where counteracting inequality or discrimination becomes the central issue. Finally, it moves down to the level of applied ethics, with a focus on...