Background: Approximately 200K transitioning service members/Veterans (TSMVs) separate from the military annually and they face elevated suicide risk levels for at least six years after separation. Suicide risk is greatest for those with the most difficulty reintegrating into civilian life roles at home, work, and in the community. Current research on the military-to-civilian reintegration process suffers from a lack of systematized measurement, longitudinal studies, and attention to modifiable factors that influence how individuals respond to disruptive life events. This CDA-1 proposal will leverage longitudinal data from a VA research repository to address these gaps by conducting the first in-depth psychometric investigation of the Military-to-Civilian Questionnaire (M2CQ), followed by an investigation of how reintegration difficulties develop over time. Further, it will evaluate DSM-5 personality functioning as a modifiable predictor of reintegration challenges and subsequent suicide risk during the transition period. Significance: Rehabilitation is relevant to the reintegration of all TSMVs, who share the goal of adapting to post- military life roles. This CDA-1 will deliver recommendations for assessing reintegration and yield critical insights for identifying TSMVs prone to reintegration difficulty. These are essential steps for detecting intervention targets that promote positive functional outcomes and disrupt suicide risk trajectories. Identifying modifiable predictors of reintegration difficulty will inform a future CDA-2 application focused on developing targeted interventions to address those factors. This CDA-1 is aligned with priorities outlined by the White House’s National Strategy for Preventing Veteran Suicide and the VA’s National Roadmap to Empower Veterans and End Suicide. This project is also responsive to RR&D’s request for suicide prevention research that involves Veterans not currently receiving VA healthcare. Innovativeness: The proposed research represents the first comprehensive psychometric evaluation of the M2CQ in a longitudinal sample of TSMVs. Second, by incorporating the construct of personality functioning, it suggests new ways of thinking about reintegration difficulty and its relationship to suicide risk. Third, the longitudinal design includes pre- and post-transition timepoints, which provide a unique opportunity to directly evaluate temporal changes in reintegration difficulties as a function of co-occurring personality functioning problems and the major life event of separation from service. Specific Aims: Aim 1 is (a) to investigate the M2CQ’s structural validity, reliability, and item-level properties; (2) evaluate how well M2CQ scores/items detect meaningful and expected change in functioning over time; (3) to determine whether total scores and items have particularly strong associations with negative clinical outcomes. Aim 2 is to evaluate personality functioning as a potentially modifiable individual...