The use of animals in scientific research is highly controversial. Older justifications, which referred to an immense gulf between human beings and other animals, can no longer be sustained in the face of a large body of scientific evidence concerning the similarities between human beings and other animals. The probability is very high that they are like us in many important ways, including in having a capacity to suffer. Because animals may suffer during research, their use must be justified. An appropriate justification will require that researchers can demonstrate that the expected benefits of the research, in terms of pure knowledge and medical applications, outweigh the suffering imposed. However, while the infliction of suffering on animal models must meet stringent conditions, research which involves the (painless) death of animals is often easier to justify, since few animals other than human beings possess the psychological capacities required to care about their future.