The proposed definitions for refractory migraine (RM) and refractory chronic migraine (R-CM) comprise 5 key components that must be operationalized for epidemiologic research. Persons with RM or R-CM must meet the second edition of the International Classification of Headache Disorders criteria for migraine or chronic migraine. They must experience significant interference with function or quality of life due to headaches. This interference must be present despite adequate treatment in 3 domains: modification of triggers and lifestyle factors, acute medication, and preventive medicines. The epidemiologic data which address these 5 components will be reviewed herein though specifically designed studies will be required to fully explore RM and R-CM. In addition, 2 “modifiers” of RM and R-CM have been proposed; one addresses medication overuse and the other considers disability based on a Migraine Disability Assessment score of 11 or greater. The epidemiology of these modifiers is discussed.