OBJECTIVE: The prevalence, disability, progression, and treatment needs associated with chronic migraine (CM) mandate epidemiological, clinical, and basic research to better understand the clinical course of this disorder and to facilitate development of more effective therapies. Such efforts have been significantly impeded by lack of agreement within the headache specialist community of the most appropriate diagnostic criteria for CM. METHODS: This paper reviews the pertinent nosological literature and extensive field testing already performed. RESULTS: We recommend that the International Classification of Headache Disorders-3β criteria for CM be modified. We would remove the need for 5 prior migraine attacks and would replace "Headache considered by patient to be onset migraine and relieved by a triptan or an ergotamine derivative" with "criteria A and B for 1.5 probable migraine." CONCLUSIONS: The proposed criteria are guided by the aims of accurately characterizing patients with migraine who develop primary chronic daily headache, reflecting the large numbers of patients with CM in clinical practice, and facilitating research into a disorder that is an academic and clinical priority.