The supposed synonymity of obsessions and intrusive thoughts is critically examined. It is suggested that there may be distinct differences in the appraisal of classic intrusive thoughts which feature aggressive or sexualised content and the appraisal of contamination fears. This is significant for both the current definition of obsessions and to the cognitive model proposed by Salkovskis (1985, 1989). Following a review of the research into the relationship between intrusive thoughts and obsessions, Salkovskis's cognitive-behavioural model of obsessive-compulsive disorder (OCD) (Salkovskis, 1985, 1989) is examined and two key papers, which are often cited to support the view that intrusive thoughts in the normal population are synonymous with obsessive thoughts as experienced by OCD patients, are critically appraised. Evidence from a number of phenomenological studies is also considered as an alternative source of information about the nature of obsessions in the clinical population. The development of a new measure, the Obsessive Thoughts Appraisal Scale is reported. This measure was used with both clinical and non-clinical samples (n = 109). It was found to have adequate psychometric properties. The results of the hypothesis testing indicate that judgements of ego-syntonic are not restricted to some small number of patients suffering from overvalued ideation, but rather ego-syntonic appraisals of some obsessions appears to be widespread but closely related to the individual's degree of obsessionality. Furthermore, obsessions concerning contamination were found to be significantly less ego-dystonic than those that concerned aggressive or sexually intrusive thoughts. Suggestions are made for both clinical practice and future research.