In 1995 the Obsessive Compulsive Cognitions Working Group initiated a collective process to develop two measures of cognition relevant to current cognitive-behavioural models of OCD. An earlier report (Behav. Res. Therapy, 35 (1997) 667) describes the original process of defining relevant domains. This article describes the subsequent steps of the development and validation process: item generation, scale reduction, and initial examination of reliability and validity. Two scales were developed. The Obsessive Beliefs Questionnaire consists of 87 items representing dysfunctional assumptions covering six domains: overestimation of threat, tolerance of uncertainty, importance of thoughts, control of thoughts, responsibility, and perfectionism. The Interpretation of Intrusions Inventory consists of 31 items that refer to interpretations of intrusions that have occurred recently. Three of the above domains are represented: importance of thoughts, control of thoughts, and responsibility. The item reduction and validation analyses were conducted on clinical and non-clinical samples from multiple sites. Initial examination of reliability and validity indicates excellent internal consistency and stability and encouraging evidence of validity. However, high correlations indicating overlap between some of the scales, particularly importance of thoughts, control of thoughts, and responsibility will need to be addressed in subsequent empirical and theoretical investigations.