These practice parameters describe the assessment and treatment of obsessive-compulsive disorder based on a detailed literature review and expert consultation. Obsessive-compulsive disorder is a disorder of heterogeneous origin characterized by intrusive thoughts or compulsive urges or behaviors that are distressing, time-consuming, or functionally impairing. In children and adolescents, the disorder often is accompanied by a wide range of comorbidity, including mood, anxiety, attentional, and learning difficulties, and/or tic disorder. These parameters describe the relevant areas of assessment, especially symptomatology, onset, and course, other associated psychopathology, and developmental, family, and medical history (including postinfectious onset or exacerbations). Two modalities have been systematically assessed and empirically shown to ameliorate core symptoms: cognitive-behavioral therapy (primarily exposure/response prevention) and serotonin reuptake inhibitor medication. Data regarding the indications, efficacy, and implementation of these modalities are reviewed. Because OCD frequently occurs in the context of other psychopathology and adaptive difficulties, additional individual and family psychotherapeutic, pharmacological, and educational interventions often are necessary. Treatment planning guidelines are provided. J. Am. Acad. Child Adolesc. Psychiatry, 1998, 37(10 Supplement):27S–45S.