Obsessive-compulsive disorder (OCD) affects approximately 1 person in 40 and causes great morbidity and suffering worldwide. While much about this protean disorder remains unclear, our understanding has advanced along many fronts in recent decades, and evidence-based treatments can produce benefit in a majority of sufferers. This text brings together experts in all aspects of OCD, including clinical presentation, current psychological, genetic, and neurobiological understanding of its etiology and pathophysiology, and psychotherapeutic, pharmacological, and anatomically targeted treatments. OCD-related disorders and common comorbidities and their relationship to OCD itself are also discussed, as are theoretical and sociological issues. It is hoped that this text will provide a comprehensive introduction to the field for students, scientists and clinicians. By bringing together many different perspectives on OCD, we aim to encourage cross-disciplinary understanding, research, and advances in clinical care.