Post-traumatic stress disorder (PTSD) is a debilitating psychiatric disorder with an estimated genetic component accounting for 30%–40% of the variance contributing to risk for the disease. This chapter starts with a review of the biological hypotheses and related genetic mechanisms currently proposed to be associated with PTSD and trauma-related disorders. It will follow with a description of the state-of-the-art on the methodologies and their application to map genetic loci and identify biomarkers associated with PTSD. Finally, we will review the latest results from genome-wide association studies of genetic variants as well as those derived from the emerging fields of epigenetics and gene expression.