Perspectives on the use of animals for biomedical research are changing in fundamental and profound ways. The longtime standoff between scientists who conduct research on animals in the hope of advancing medical knowledge and people who object to that work for being immoral is beginning to yield to efforts to find common ground. New initiatives are seeking to greatly reduce the number of animals used, particularly in toxicology testing. Attention has also focused on the ethical justification and scientific necessity of research on chimpanzees and other primates. This special report describes the state of the debate over the use of animals in biomedical experiments – the ethical issues, the scientific arguments for and against using animals in particular kinds of studies, the availability of alternative models that might replace whole animals in some research, and the ways U.S. laws that govern animal experimentation can be amended to reduce unnecessary animal suffering.