Sleep/wake complaints, and specifically insomnia, are some of the more common problems encountered in the outpatient setting. Despite its prevalence, few clinicians are experts at diagnosing and treating this entity. Additionally, diagnosis and treatment of insomnia is a time-intensive process (often the initial interview takes at least 1 h, depending on the complexity of the insomnia). With a conservative estimate of the annual cost of insomnia between dollar 92.5 and dollar 107.5 billion dollars, it is becoming clear that insomnia has significant medical and public health implications. A problem that has hampered insomnia research is the lack of a standard definition of insomnia for use in research, as well as guidelines for assessment. In recent years, there have been important advances in the classification, evaluation, and treatment of insomnia with efforts to establish greater consensus in how to define and measure insomnia. Cognitive behavioral and pharmacologic therapies have been shown to be effective treatment approaches. Insomnia is a complex entity, often multifactorial in its etiology; and as research and clinical guidelines are established and validated (leading to better data interpretation), continued enhancement of our understanding of this disorder is expected.