Obsessive compulsive spectrum disorders (OCSDs) are conditions that, while not meeting diagnostic criteria for obsessive compulsive disorder (OCD), share many similar symptoms. The past decade has seen a tremendous growth in knowledge on the nature, treatment, assessment, and basic science of OCSDs. This book reviews the latest research on OCD and OCSDs and provides evidence-based guidance for assessment and treatment. Several different conditions are covered, including hoarding disorder; body dysmorphic disorder; hair pulling, skin picking, and other body-focused repetitive behaviors; Tourette disorder and tics; hypochondriasis; as well as OCD. Importantly, the book takes a lifespan perspective, with specific attention given to the unique aspects of OCSDs across different age groups. Both psychological and pharmacological treatments are reviewed, as well as comorbidities and other complications. Two special chapters review the neurobiological and behavioral genetic support for the obsessive compulsive spectrum.