In this paper we review the assessment and measurement of normal unwanted intrusive thoughts, images, and impulses that are considered the basis of clinical obsessions. After highlighting some difficulties with how the definition of cognitive intrusion has been applied to the development of assessment measures, we evaluate the construct validity of a number of retrospective self-report instruments such as the Intrusive Thoughts Questionnaire, Cognitive Intrusions Questionnaire, and Obsessional Intrusions Inventory, as well as interview and diary procedures. Measures of personal responsibility and meta-cognitive beliefs, which are still in the developmental phase, are also discussed. We conclude with a number of recommendations and areas of further research which would strengthen the construct validity of measures of intrusive thoughts and related constructs.