The prevailing view is that individuals with obsessive-compulsive disorder (OCD) are able to think rationally about their obsessive concerns and are thus able to recognize them as senseless. However, clinical observations indicate that at least some obsessive-compulsives do not regard their symptoms as unreasonable or excessive, and their ideas have been characterized as overvalued or delusional. In the present paper the concepts of obsessions, overvalued ideas, and delusions are discussed and compared, and the available studies of insight among obsessive-compulsives are reviewed. It is concluded that obsessive-compulsive ideas can not satisfactorily be dichotomized according to patients' insight, and that the notion of a continuum of strength of obsessive-compulsive beliefs is more appropriate. The relationship between degree of obsessive-compulsive conviction and outcome of therapy remains unclear. Methodological issues that complicate our understanding of OCD are considered, and theories of delusions are examined in relation to their development in OCD.