Violations of human rights in wars, civil conflicts, and brutal repression mounted by governments against their own citizens often have profound consequences to individual and public health and may, in turn, produce humanitarian crises. The skills of physicians, medical and forensic scientists, and other health workers are uniquely valuable in human rights investigations and documentation, producing evidence of abuse more credible and less vulnerable to challenge than traditional methods of case reporting. Only in recent decades, however, have physicians organized specifically to meet this responsibility. This article presents case studies from the field missions of Physicians for Human Rights to illustrate the investigation and documentation of violations of medical neutrality, refugee health crises, the use of indiscriminate weapons, torture, deliberate injury and rape, and mass executions. Participation of health workers in the defense of human rights now includes investigation and documentation of health effects in threatened populations as well as individual victims.