Insomnia is highly co-morbid with psychiatric disorders, making it a frequent issue in treatment planning in psychiatric clinics. Research has also shown that although insomnia may originally precede or be a consequence of a psychiatric disorder, insomnia likely becomes semi-independent, and may exacerbate those disorders if it is not addressed, leading to reduced treatment response. Cognitive behavioural therapy for insomnia (CBT-I) is now recommended as the first line of treatment of primary insomnia. The research reviewed below indicates that CBT-I in patients with co-morbid depression, anxiety, post-traumatic stress disorder (PTSD), and substance abuse disorders is generally effective for insomnia and sometimes the co-morbid disorder as well. Although more research is needed before definitive recommendations can be made, it appears as though CBT-I is a viable approach to treating the patient with co-morbid insomnia and psychiatric disorders.