The Cognitive Style Questionnaire (CSQ) measures the cognitive vulnerability factor featured in the hopelessness theory of depression. The CSQ has been used in over 30 published studies since its inception, yet detailed information about the psychometric and validity properties of this instrument has yet to be published. In this article, we describe the development of the CSQ and review reliability and validity evidence. Findings to date using college samples, indicate that the CSQ is a reliable measure of cognitive vulnerability with a high degree of construct validity.