Secondary insomnia is defined as difficulty initiating or maintaining sleep that occurs as a consequence of another primary medical or psychiatric disorder. This distinction from primary insomnia has been thought to be important from a diagnostic standpoint so that treatment for secondary insomnia can appropriately target the primary disorder. By convention, treating insomnia directly has only been recommended with primary insomnia. However, a number of recent studies of cognitive-behavioral treatment for a wide range of secondary insomnias have demonstrated that this approach is an effective treatment in this population. These results are not predicted by the traditional conceptualization of secondary insomnia. Results of epidemiological research also argue against viewing insomnia that is comorbid with another disorder as being caused by that disorder. A revised model is needed to understand insomnia that is comorbid with medical and/or psychiatric illness, in addition to re-evaluating standard treatment protocols for comorbid insomnia.