Chronic migraine is a common and disabling complication of migraine with a population prevalence of about 2%. Emerging evidence suggests that episodic migraine and chronic migraine differ not only in degree, but also in kind. Compared with patients with episodic migraine, those with chronic migraine have worse socioeconomic status, reduced health-related quality of life, increased headache-related burden (including impairment in occupational, social, and family functioning), and greater psychiatric and medical comorbidities. Each year, approximately 2.5% of patients with episodic migraine develop new-onset chronic migraine (ie, chronification). Understanding the natural disease course, improving treatment and management, and preventing the onset could reduce the enormous individual and societal burden of chronic migraine, and thus, have become important goals of headache research. This review provides a summary of the history of nomenclature and diagnostic criteria, as well as recent studies focusing on the epidemiology, natural history, and burden of chronic migraine.