Anxiety disorders occur frequently, begin at an early age, significantly impair patients' lives, and are often associated with other comorbid conditions. Despite impressive improvements in the treatment of anxiety disorders in the last decade, the mechanisms leading to comorbidity and the processes responsible for therapeutic change remain unclear. Within this context, clear and precise definitions of the putative active ingredients are needed in order to further improve treatments. This article reviews these critical issues and sets the stage for the other articles in this special issue.