Obsessive compulsive and related disorders (OCRDs) in children and adolescents represent a cluster of conditions that significantly interfere in the lives of sufferers and their families. These disorders involve repetitive behaviors and often a preoccupation with distressing, obsessional thoughts. OCRDs include obsessive–compulsive disorder (OCD), body dysmorphic disorder (BDD), hoarding disorder, trichotillomania, and excoriation disorder. The severity, functional impairment, and associated health conditions of these disorders call for timely evidence-based assessment and treatment. Evidence-based assessments include structured and semistructured interviews. Interviews allow for the assessment of symptoms, comorbid conditions, and differential diagnoses. Evidence-based psychological treatment for OCD and BDD in youth involves cognitive behavioral therapy with exposure and response prevention; research is required to determine evidencebased assessment and treatments for less studied OCRDs; identify factors that predict poorer response to evidence-based treatment; develop approaches to augment evidencebased treatments for nonresponders; and further the reach of empirically supported treatment.