U.S. military forces are increasingly involved in a variety of multinational peacekeeping and humanitarian assistance missions. How well combat-trained units and soldiers adapt to these new roles will determine U.S. success in such operations, as well as the future health and readiness of the force. In preparing soldiers for such missions, it is critical that leaders and health care providers have a clear understanding of the nature of the stressors they are likely to encounter. This report summarizes findings from a longitudinal, descriptive case study of a U.S. Army medical unit performing a peacekeeping mission in the former Yugoslavia. The goal of the investigation was to identify key sources of stress and to delineate the effect of these stressors on the health, morale, and mental readiness of soldiers. Findings suggest a range of psychological stressors that varies somewhat across operational phases of a peacekeeping mission. Furthermore, the degree of stress experienced in various areas correlates significantly with depression, psychiatric symptoms, and low reported morale. The range of stressors is reduced and summarized in a conceptually derived model of five underlying dimensions of psychological stress salient to soldier adaptation in peacekeeping operations: isolation, ambiguity, powerlessness, boredom, and danger/threat. This model provides a useful heuristic for organizing thinking about stress in peacekeeping operations and leads to several recommendations for "countermeasures" that organizational leaders can take to maintain soldier psychological readiness during peacekeeping operations.