The growing recognition and occurrence of traumatic exposure in the general population has given increased salience to the need to understand the concept of resilience. More than just the “flip side” of a risk factor, the notion of resilience encompasses psychological and biological characteristics, intrinsic to an individual, that might be modifiable and that confer protection against the development of psychopathology in the face of stress. In this review, we provide some perspective on the concept of “resilience” by examining early use of the term in research on “children at risk” and discuss the relationship between risk and resilience factors. We then review psychological and biological factors that may confer resilience to the development of posttraumatic stress disorder (PTSD) following trauma, examine how resilience has been assessed and measured, and discuss issues to be addressed in furthering our understanding of this critical concept going forward. Depression and Anxiety 24:139–152, 2007. © 2006 Wiley-Liss, Inc.