Sleep disorders, both insomnia and hypersomnia, are commonly associated with various co-morbid conditions, including general medical and neurologic disorders, psychiatric illnesses, and secondary or symptomatic restless legs syndrome/periodic limb movements in sleep. Diagnosis of the co-morbidity is the first step in treatment, followed by an assessment of the sleep disturbance. This begins with a complete history and physical examination, followed by laboratory testing such as polysomnography, multiple sleep latency testing, and actigraphy. The treatment of sleep disorders caused by co-morbid conditions is discussed under seven categories, including the use of general measures (e.g., sleep hygiene, encouraging patients to develop good sleep habits, medication), treatment of the co-morbid condition, treatment of insomnia, treatment of excessive daytime somnolence, treatment of parasomnias, treatment of sleep-wake schedule disorders, and treatment of secondary or symptomatic restless legs syndrome.