Posttraumatic stress disorder (PTSD) is a prevalent anxiety disorder marked by behavioral, physiologic, and hormonal alterations. PTSD is disabling and commonly follows a chronic course. The etiology of PTSD is unknown, although exposure to a traumatic event constitutes a necessary, but not sufficient, factor. A twin study of Vietnam veterans has shown significant genetic contribution to PTSD. The fact that PTSD's underlying genotypic vulnerability is only expressed following trauma exposure limits the usefulness of family-based linkage approaches. In contrast to the other major psychiatric disorders, large studies for the search of underlying genes have not been described in PTSD to date. Complementary approaches for locating involved genes include association-based studies employing case-control or parental genotypes for transmission dysequilibrium analysis and quantitative trait loci studies in animal models. Identification of susceptibility genes will increase our understanding of traumatic stress disorders and help to elucidate their molecular basis. The current review provides an up-to-date outline of progress in the field of PTSD.