This article reports on the validation of the Obsessive Beliefs Questionnaire (OBQ) and Interpretations of Intrusions Inventory (III) developed by the Obsessive Compulsive Cognitions Working Group (OCCWG) to assess the primary beliefs and appraisals considered critical to the pathogenesis of obsessions. A battery of questionnaires that assessed symptoms of anxiety, depression, obsessive-compulsive symptoms and worry was administered to 248 outpatients with a DSM-IV diagnosis of Obsessive-Compulsive Disorder (OCD), 105 non-obsessional anxious patients, 87 non-clinical adults from the community, and 291 undergraduate students. Tests of internal consistency and test-retest reliability indicated that the OBQ and III assessed stable aspects of OC-related thinking. Between-group differences and correlations with existing measures of OC symptoms indicated that the OBQ and III assess core cognitive features of obsessionality. However, the various subscales of the OBQ and III are highly correlated, and both measures evidenced low discriminant validity. The findings are discussed in terms of the relevance and specificity of cognitive constructs like responsibility, control and importance of thoughts, overestimated threat, tolerance of uncertainty and perfectionism for OCD.