Individuals differ strongly in their vulnerability to develop posttraumatic stress disorder (PTSD) in the aftermath of traumatic stress. This review on genetic risk factors in PTSD etiology employs the perspective of a psychobiological model, which proposes that intrusive memories, the core PTSD symptom, result from the formation of an associative neural fear network, which stores sensory-perceptual representations of traumatic memories. The current state of research on the genetics of PTSD, as well as common challenges, is presented in light of this framework. Because cumulative trauma exposure increases the fear memory strength, a standardized assessment of traumatic load and the investigation of Gene x Environment interactions are recommended. The investigation of genes involved in long-term memory formation, genome-wide association studies, pathway analyses, and the interplay of genetic and epigenetic factors could contribute to a deeper understanding of the molecular pathways involved in the formation and modification of the fear network.