This chapter provides some background about post-traumatic stress disorder (PTSD) and reviews the evidence for the role of genetic factors as a diathesis for PTSD. There are three approaches that have traditionally been used to investigate the presence of genetic influences on psychopathology: family studies, twin studies, and adoption studies. There have been a number of case-control association studies conducted to try to identify genetic variants that are related to a vulnerability to develop PTSD following exposure to trauma. There have been a number of twin studies that have examined possible genetic influences that contribute to the comorbidity of PTSD with other mental disorders. An area that is likely to play an increasingly important role in understanding how genes influence psycho-pathology in general and PTSD in particular is epigenetics. The cutting-edge research design for gene discovery is currently the genome-wide association studies (GWAS).