Recent concern about the way humans treat animals has spawned a powerful social and political movement driven by the conviction that humans and certain animals are similar in morally significant ways, and that these similarities oblige humans to extend to those animals serious moral consideration, including rights. Though animal welfare movements, concerned primarily with humane treatment of pets, date back to the 1800s, modern animal rights activism has developed primarily out of concern about the use and treatment of domesticated animals in agriculture and in medical, scientific, and industrial research. The rapid growth in membership of animal rights organizations testifies to the increasing momentum of this movement. The leading animal rights group today, People for the Ethical Treatment of Animals (PETA), was founded in 1980 with 100 individuals; today, it has over 300,000 members. The animal rights activist movement has closely followed and used the work of modern philosophers...