Although previous reviews have established strong co-occurrences between both trauma and posttraumatic stress disorder (PTSD) with substance use disorders (SUDs; e.g., Stewart, 1996), research on this issue has dramatically increased over the past 2 decades. This chapter provides an up-to-date review of the extant literature that establishes these relationships across general population and treatment-seeking samples, gender, and developmental stage. We specifically examine the evidence for relations between trauma exposure, PTSD, and SUDs by trauma type (e.g., disaster vs. combat) and by substance (e.g., alcohol vs. cannabis) in an attempt to determine whether the relations are specific or general across traumata and drug type. The intent of this opening chapter is to set the stage for the chapters that follow by establishing the scope of the problem. We conclude with a consideration of possible directions for future research on the epidemiology of this form of comorbidity. (PsycINFO Database Record (c) 2015 APA, all rights reserved)