This thesis addresses the moral permissibility of illegal acts of animal liberation in the form of civil disobedience, acts of rescue, and acts of sabotage. Animal liberation movements have been the subject of much media and political attention, with particular focus on use liberationist strategies of intimidation, vandalism, and harassment. Governments have mobilised state apparatus in surveillance, infiltration, and investigation, and have characterising radical activism as 'terrorism'. The variety of illegal activities aimed at preventing harm to non-human animals, particularly those involving violence towards property or persons, have often been classified together under the term 'animal liberation' and assumed to be wrong. I argue that the assumption of wrongness is questionable because it fails to give significant weight to the justification for acts of animal liberation. I pose the question as to whether and what illegal practices of animal liberation are ethically justifiable. I begin by arguing that non-human animals are worthy of moral consideration for their own sake, because their sentience above a basic level, particularly their capacity to suffer, gives moral agents reasons to acknowledge and respect their goods. Following this, I defend the claim that liberal democratic states that fail to treat animals living within them with respect are unjust. This injustice provides a justification for civil disobedience on behalf of non-human animals. Following this, I argue that beings worthy of moral consideration are owed positive duties of aid and easy rescue and I extend third-party intervention theory to non-human animals under threat from humans. I explore the limits to the duties of aid and intervention, using principles drawn from those of humanitarian intervention to identify duty bearers, and I weigh those duties against duties to fellow citizens and the state.