Military Veterans are at risk of developing posttraumatic stress disorder (PTSD) following exposure to combat while on deployment. Up to 30 percent of Veterans suffer from PTSD and many do not seek treatment due to the perceived stigma of seeking mental health services, distrust of the mental health profession, or perception that treatment is inadequate. Left untreated, PTSD can cause significant social, emotional, and financial hardships for the Veteran, family, and community. Animal assisted interventions (AAI) have been shown to be a successful, acceptable alternative and complementary therapy for PTSD. A literature review yielded 16 studies of AAI for Veterans with PTSD. Pinnipeds, canines, and equines were the animals used for AAI. Although the methods, lengths, and dose of therapy differed across studies, AAI were found to decrease level of PTSD, anxiety, and depression and increase in quality of life and social support. A qualitative, thematic analysis of 116 Veteran riding diary entries following therapeutic horseback riding (THR) revealed five themes: (a) social interaction, (b) self-awareness, (c) mood, (d) relationship development and bonding, and (e) positive experience. The central theme was that THR decreases the symptoms of PTSD. As research continues to support the benefits of AAI, it is essential to also study the health and welfare of the animals used. A quantitative analysis of N = 5 Veterans with PTSD participating in THR examined the relationship between level of PTSD and physiologic and behavioral indicators of stress. No significant correlation was found between level of PTSD and equine cortisol levels, nor level of PTSD and equine behavior scores (EBS). Level of PTSD did not significantly decrease across THR sessions; however, it did move in the expected direction. Although mean equine cortisol levels increased across sessions, levels remained within normal range. Three was no significant increase in mean EBS across THR sessions. Results of this dissertation support the use of AAI in Veterans with PTSD.