Insomnia is the most common sleep problem affecting nearly one-third of the population as either a primary or comorbid condition. Insomnia has been defined as both a symptom and a disorder, and is characterized as sleep that is chronically unrestorative or poor in quality often due to difficulty in initiating sleep, in maintaining sleep, or with waking up too early. Insomnia results in some form of daytime impairment in the patient's normal activites. Although the exact pathophysiology of insomnia is poorly understood, it is often believed to arise from a state of hyperarousal in multiple neurophysiological and/or psychological systems. Population-based studies suggest that while about one-third of the general population complains of sleep disturbance, only 10-15 percent has associated symptoms of daytime functional impairment, and even fewer, only 6-10 percent have impairments sufficient for the diagnostic criteria of insomnia. The cornerstone of the insomnia evaluation and diagnosis is a comprehensive history obtained by the clinical interview with patient and/or family. Additional assessment tools, such as sleep diary or log, various questionnaires, actigraphy, and multichannel polysomnography (PSG) have been used as an aid to diagnosis, although many are limited in their validation. Insomnia causes a significant burden of medical, psychiatric, societal consequences on the individual and societal level. Clinicians in either primary settings or specialized clinics should have knowledge to manage insomnia with confidence.