There is substantial evidence from studies in humans and animal models for a role of the endocannabinoid system in the control of emotional states. Several studies have shown an association between exposure to trauma and substance use. Specifically, it has been shown that there is increased prevalence of cannabis use in post-traumatic stress disorder (PTSD) patients and vice versa. Clinical studies suggest that PTSD patients may cope with their symptoms by using cannabis. This treatment-seeking strategy may explain the high prevalence of cannabis use among individuals with PTSD. Preliminary studies in humans also suggest that treatment with cannabinoids may decrease PTSD symptoms including sleep quality, frequency of nightmares, and hyperarousal. However, there are no large-scale, randomized, controlled studies investigating this specifically. Studies in animal models have shown that cannabinoids can prevent the effects of stress on emotional function and memory processes, facilitate fear extinction, and have an anti-anxiety-like effect in a variety of tasks. Moreover, cannabinoids administered shortly after exposure to a traumatic event were found to prevent the development of PTSD-like phenotype. In this article, we review the existing literature on the use of cannabinoids for treating and preventing PTSD in humans and animal models. There is a need for large-scale clinical trials examining the potential decrease in PTSD symptomatology with the use of cannabis. In animal models, there is a need for a better understanding of the mechanism of action and efficacy of cannabis. Nevertheless, the end result of the current clinical and preclinical data is that cannabinoid agents may offer therapeutic benefits for PTSD.