Is insomnia a clinical entity in its own right or is it simply a symptom of an underlying medical or psychological disorder? The widely held view among many clinicians and researchers is that insomnia is secondary to or an epiphenomenon of a 'primary' medical or psychological disorder. Consequently, insomnia 'symptoms' have tended to be trivialized or ignored. This paper aims to highlight the assumptions and implications of distinguishing between 'primary' and 'secondary' insomnia and reviews the evidence for the distinction by considering (1) issues relating to the diagnosis and classification of insomnia, (2) whether insomnia is a symptom of other medical and psychological disorders, (3) whether insomnia is comorbid with other disorders, (4) whether insomnia is 'secondary' to other disorders, and (5) whether insomnia occurs in the absence of comorbidity. It is concluded that viewing insomnia as a symptom or epiphenomenon of other disorders can be unfounded. This view may deprive many patients of treatment, which might not only cure their insomnia, but may also reduce symptoms associated with the assumed 'primary' disorder. Finally, directions for future research to further illuminate the relationship between insomnia and comorbid disorders are discussed.