Introduction to the Special Issue: Conceptualization, Assessment, and Treatment of Obsessive-Compulsive Disorder and Co-Occurring Conditions.

Obsessive-compulsive disorder (OCD) has a lifetime prevalence of 1%-2% and typically presents as a chronic condition with significant functional impairment. Comorbidity with OCD is the norm, with 90% of individuals with OCD also meeting diagnostic criteria for a co-occurring condition. Co-occurring conditions can complicate the conceptualization, assessment, and treatment of OCD, such as by intensifying existing symptoms, obscuring differential diagnosis of phenotypically and functionally similar symptoms, and interfering with cognitive behavioral treatment. This two-part special issue reviews extant literature and provides expert advice on conceptualizing, assessing, treating, and researching OCD with co-occurring conditions of depression, posttraumatic stress disorder, eating disorders, schizophrenia, hoarding disorder, panic disorder, obsessive-compulsive personality disorder, and illness anxiety disorder.