The purpose of the current study was to examine a process model of combat-related and mental-illness related processes that explain increased likelihood of Posttraumatic Stress Disorder (PTSD). This dissertation proposed the development of PTSD may occur due to cultural, social, and self-related pathways associated with veterans’ dual encounters with combat (i.e., severity) and mental illness symptoms. Participants were 195 military veterans recruited from multiple sites and strategies to maximize sample size and representation. Participants were asked to complete several self-administered assessment inventories, including: the Posttraumatic Stress Disorder Checklist-Military, the Trauma Symptom Checklist, the Combat Experiences scale, the Self-Stigma of Mental Illness Scale, an adapted version of the Iraq War Attitude Scale, a perceptions scale, an adapted version of the Likelihood of Disclosure Scale, the Unit Support Scale, the Post-Deployment Support Scale, the UCLA Loneliness Scale (Version 3), as well as covariates that included demographics and details of military service (e.g., deployment information). Overall, results revealed that the impaired social support indicator of social isolation was linked to PTSD, whereas impaired unit support and impaired postdeployment support were not predictive of PTSD. Results also revealed that it is the cultural stereotypes and stigma associated with military and war but not of mental illness that plays a role in social isolation and subsequently PTSD. Overall, evidence supports the combined explanations of combat-related processes and mental illness processes in understanding likelihood of PTSD.