This chapter introduces topical issues in the ethical debate on speciesism. It does so against a background of the history of the debate and with an emphasis on concerns that arise at the intersection of speciesism and science. The term speciesism was coined in the 1970s by Richard Rider and popularized by Peter Singer, who defined speciesism as “a prejudice or attitude of bias in favor of the interests of members of one’s own species and against those of members of other species.” Critiques of speciesism are undergirded by the principle that comparable interests should be given equal consideration, regardless of species-membership. To what extent the interests of humans and non-humans are comparable, however, has been a matter of contention and is partly beholden to empirical findings. Diverging opinions on this matter have given rise to a variety of more and less species-egalitarian views, often paired with rivalling accounts of “moral status”. A minority of ethicists have explicitly defended speciesism or some version thereof, including Bernard Williams and, more recently, Shelly Kagan. Apart from the back-and-forths between critics and defenders, the speciesism debate has transformed over the last decade, in three respects. First, discourse on animal ethics has witnessed a “political turn”, moving away from analyses of moral status and towards discussions of human-animal relations in interspecies societies. Secondly, apart from anthropocentric speciesism, i.e., privileging humans over non-humans, there has been an increasing interest in non-anthropocentric speciesism, i.e., privileging some non-human species over others. Thirdly, speciesism has been endorsed as a prominent topic in the new, activist-leaning research field of Critical Animal Studies, and has recently garnered substantial attention from psychologists. As these disciplinary changes and conceptual refinements suggest, half a century after its coinage, speciesism continues to be an important scholarly and ethical concept whose history is still in the making.