Post-traumatic stress disorder (PTSD) represents a frequent consequence of a variety of extreme psychological stressors. Lists of empirically supported treatments for PTSD usually include cognitive behavioral therapy (CBT) and eye movement desensitization and reprocessing (EMDR), but nonresponse and dropout rates in these treatments often are high. We review the treatment dropout and nonresponse rates in 55 studies of empirically supported treatments for PTSD, review the literature for predictors of dropout and nonresponse, discuss methodological inconsistencies in the literature that make comparisons across studies difficult, and outline future directions for research. Dropout rates ranged widely and may have depended, at least in part, on the nature of the study population. It was not uncommon to find nonresponse rates as high as 50%. Standard methods of reporting dropout and nonresponse rates are needed for reporting outcomes. We suggest guidelines for collecting data to help identify characteristics and predictors of dropouts and nonresponders.