In the last decade, the number of publications in psychiatric genetics has nearly tripled but little attention has been paid to the role of genetic factors in the etiology of posttraumatic stress disorder (PTSD). The present review summarizes the current state of genetic research on PTSD. First, we outline information regarding genetic influences provided by family investigations and by twin studies. Second, we propose the fear-conditioning model of PTSD as a framework for the nomination of candidate genes that may be related to the disorder. Third, we review lines of evidence from three neurobiological systems involved in fear conditioning, and we summarize published investigations of genetic variants studied in association with PTSD in these three systems. Finally, we review gene-by-environment interaction research, a promising novel approach to genetic research in PTSD.