Experimental Design. The study design is a parallel group, double-blind, placebo-controlled, stratified, randomized medication treatment trial of male and female veterans who experienced "in-theater" trauma and have both a PTSD and Alcohol Use dual diagnosis. All subjects will receive 12 weeks of manualized cognitive & behavioral therapy as a standard of care. Additionally, subjects will be randomized 1:1 to receive double-blind treatment with Sertraline vs. Placebo as an adjunctive treatment. The manualized therapy provides standard cognitive-behavioral therapy (CBT) addressing alcohol use and prolonged exposure (PE) behavioral therapy targeting PTSD. Sertraline is a common treatment for PTSD and psychiatric disturbance but is hypothesized to have differential efficacy in different subtypes of alcohol drinking patients. A stratified randomization scheme will balance placebo and sertraline assignment to two groups, and a post-hoc clustering approach will be used to determine which subgroups of dual diagnosis patients may benefit from sertraline vs. placebo treatment.