In this thesis, I argue that humans and sentient animals have equal moral status in the sense that they ought to have like interests equally considered. Furthermore, they are owed strong pro tanto duties to be free from having pain inflicted upon them, having their lives ended and having their liberty restricted. I argue for these claims by developing and defending an account of moral status grounded in agency. This account takes agency, understood as the capacity to act on motivating reasons, to be the necessary and sufficient condition for moral status. Further, I argue that agency is sufficient to have interests in liberty, continued existence and freedom from pain. As such we pro tanto wrong agents when we frustrate these interests. I show that sentient beings necessarily possess agency in the relevant sense, because the best account of the nature of sentience, entails that sentient beings have the psychological resources to form and act upon motivating reasons. Thus, I argue that sentient animals must possess interests in liberty, continued existence and freedom from pain, just as autonomous agents do. Therefore we should take all agents, regardless of further facts about their abilities, to possess equal moral status and be owed pro tanto duties to be free from having pain inflicted upon them, having their lives ended and having their liberty restricted.