This randomized trial will compare a novel treatment, Acceptance of the Behavioral Changes to Treat Insomnia (ABC-I) to Cognitive Behavioral Therapy for Insomnia (CBT-I) among Veterans with comorbid Post-Traumatic Stress Disorder (PTSD) and insomnia disorder. ABC-I combines the behavioral components of CBT-I with components of another behavioral therapy (Acceptance and Commitment Therapy) and has been shown to improve treatment adherence. The study objectives are: 1) to evaluate the benefits of ABC-I in reducing post-traumatic stress disorder (PTSD) symptoms among Veterans with comorbid PTSD and insomnia disorder compared to CBT-I, and 2) to evaluate the effectiveness of ABC-I in improving insomnia symptoms and sleep quality among Veterans with comorbid PTSD and insomnia disorder as compared to CBT-I. Veterans with insomnia and comorbid PTSD who receive care at Sepulveda and West Los Angeles facilities will be recruited for the study. Those who pass an initial eligibility screen will be enrolled and written informed consent will be obtained. A baseline assessment will be completed that includes measures of sleep, PTSD, and quality of life. Veterans who meet all eligibility criteria will be randomly assigned to the ABC-I (n=100) or CBT-I (n=100) treatment. Both treatments will be provided in 5 one-on-one sessions by a trained instructor who is supervised by a behavioral sleep medicine specialist. All randomized participants (n=200) will have 3 follow-up assessments (post-treatment, 3-months, and 6-months after randomization). The follow-up assessments will collect information on PTSD symptoms, insomnia symptoms and sleep quality.