Operation Unified Assistance was the United States' historic response to the tsunami that struck Southeast Asia on December 26, 2004. The USNS Mercy participated in Operation Unified Assistance from January to March 2005. Stationed off the coast of Sumatra, Indonesia, the Mercy's crew consisted of uniformed members of the U.S. Navy and U.S. Public Health Service and civilian members of Project HOPE. The Mercy's participation in Operation Unified Assistance provided a platform and context to study disaster-related psychological processes. Observations of crew members aboard the Mercy suggested similar psychological processes for disaster victims and responders. Responder risk and protective factors; disaster responders as potential psychological casualties; responder selection, preparation, and intervention strategies; and the hospital ship as a research platform are discussed. Such issues are generally relevant to the interaction between civilian volunteers and professional disaster responders.