Many questions remain unanswered with regard to our understanding of insomnia. Although it is generally believed that 10% to 15% of the adult population suffers from chronic insomnia, and an additional 25% to 35% have transient or occasional insomnia, prevalence estimates vary because of inconsistent definitions and diagnostic criteria. The elderly in particular are affected by insomnia, and it has been shown that women are more likely to have sleep difficulties than men. Although insomnia can be a primary condition, and can coexist with other disorders or be considered secondary to these disorders, the mechanisms producing it are not clearly defined. Additionally, the relationship between insomnia and other disease states is not always clear because it is often not possible to determine the cause-and-effect relationship between disorders. Epidemiologic studies show that abnormal sleep patterns predict lower life expectancy, and that people with insomnia are more likely to develop affective disorders, substance abuse, and other adverse health outcomes. This article will provide an overview of insomnia, its prevalence and epidemiology, and guidelines for clinical assessment.