What criteria can we legitimately use to judge moral worth? What morally relevant differences or similarities exist between human and nonhuman animals? Do nonhuman animals deserve a moral status identical in all relevant ways to the moral status we attribute to other human beings? In our moral deliberations should we regard the interests of nonhuman animals as being of equal importance to human interests? Historically, with a few notable exceptions, questions such as these were either not asked or were answered unjustly, in ways which reflected the strong species bias of the philosophers doing the asking. For roughly the past two thousand years our moral thinking has focused primarily on humanity, with little serious attention given to the idea that nonhuman animals may also warrant moral concern. It was not until the 1970s that the question of moral status for nonhuman animals became a focal point of our moral thinking, and that animal ethics was acknowledged as a legitimate subject of serious philosophical inquiry. This change of attitude towards animal ethics and the issues contained therein was largely inspired by the 1975 publication of Peter Singer's seminal work Animal Liberation, which revolutionized thinking about nonhuman animals both inside and outside academia. Thus, my thesis begins with an evaluation of Peter Singer's case for animal liberation. While I support Singer's overall conclusion that nonhuman animals deserve the same moral status as human beings and that we are guilty of an inconsistency if we maintain otherwise, I see his argument as too limited in some respects to be a totally satisfactory account of why nonhuman animals deserve moral status. I construct a more comprehensive argument for the moral status of nonhuman animals by examining the issue from different philosophical perspectives (specifically, philosophy of mind and phenomenology), using these perspectives where appropriate to engage more deeply with areas of controversy such as the question of killing, morally relevant criteria, differences between the human and nonhuman animal mind, and the question of varying levels of moral significance.