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, attention, 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). Only two modalities have been systematically assessed and empirically shown to ameliorate core symptoms: cognitive behavioral therapy (primarily exposure/response prevention) and serotonin re-uptake inhibitor medication. Data regarding the indications, efficacy, and implementation of these modalities are reviewed. Because of the frequent co-occurrence of other psychopathology and adaptive difficulties, additional educational, individual and family psychotherapeutic, and pharmacological interventions often are necessary. Treatment planning guidelines are provided.