The psychological effect of peacekeeping duties on 277 New Zealand Defence Force personnel was investigated using a longitudinal, cross-sectional study. Self-report data were collected in five stages from before deployment to approximately 6 months after return. Multiple measures of mental health, physical health, and stressors were used. Results revealed that the most stressful periods of the deployment, and those with the greatest effect on overall health and well-being, appear to be the predeployment and follow-up stages. These findings demonstrate the need for effective predeployment training and postdeployment debriefing and support.