Post-traumatic stress disorder (PTSD) can affect about 3.6% of the U.S. population each year, and 6.8% of the U.S. population over a lifetime. 8.4% of people in the U.S. are diagnosed with substance use disorders (SUD). Of those with a diagnosis of PTSD, a co-occurring, or comorbid diagnosis of a SUD is present in 20–35% of that clinical population. Prevalence of SUD and PTSD may increase depending on specific populations. For example, the prevalence of both PTSD and SUD is higher in combat veterans. Alcohol use disorder (AUD) is the leading cause of SUD amongst veterans who have experienced trauma. Worsening PTSD symptoms are associated with increased SUD and poor treatment response Of those with a SUD diagnosis, current PTSD is present in 25–50%, and lifetime PTSD is present in 15–40%, averaging 30% overall. Though roughly a third of all people diagnosed with SUD also have PTSD, there is not yet consistent protocol for SUD treatment centers to screen for both PTSD and SUD symptomology upon intake. The presence of both PTSD and SUD can hinder outcomes of those seeking treatment for either PTSD or SUD. Those who experience both diagnoses may generally have poorer overall functioning and worse overall well-being than each diagnosis by itself. This can manifest as being hospitalized more frequently, experiencing increased levels of legal issues, have less social support, and have a harder time retaining employment. In treatment these individuals can have high dropout rates, respond poorly to the treatment of PTSD in general, have greater levels of addiction severity, and shorter periods of remission for substance use treatment. Etiological theory Each of the subsequent theories about the causal link between PTSD and SUD have varying levels of empirical support. These etiological theories are not mutually exclusive, and features of more than one can be present for an individual with dual diagnoses of SUD and PTSD. No one clear etiological link has been established between SUD and PTSD. Susceptibility hypothesis The susceptibility hypothesis suggests that the substance use may increase the risk of PTSD developing after a traumatic event. Individuals who use substances may lack appropriate coping mechanisms to deal with daily stressors before the traumatic event, they may be less equipped than individuals who do not use substances to cope with extreme stress. Thus, these individuals may be more susceptible to developing PTSD following a traumatic event. Implication of coping strategies Coping style has