Post-traumatic stress disorder (PTSD) is alleged to be associated with high rates of concurrent psychological disorders. This paper considers the available literature regarding comorbidity in PTSD and reviews in detail the findings to date. This critical evaluation includes studies of clinical samples, veterans community surveys, and population surveys. It also presents new data drawn from 50 PTSD cases assessed at the Boston PTSD Center using a comprehensive, multidimensional diagnostic procedure. The data firmly support the notion that PTSD, regardless of the nature of the trauma, is associated with high rates of other major psychological disorders including substance abuse, major depression, and personality disorders. Despite these findings the issue of cause and effect surrounding comorbidity remains open at this time. More research is needed to determine if exposure to extreme stressors in fact produces the broad spectrum of psychopathology suggested by individuals with PTSD or whether individuals with intrinsic vulnerability are more likely to develop PTSD when exposed to extreme stressors. Implications of the findings from this review are discussed with respect to concepts in diagnosis and comorbidity.