According to the revised 2nd Edition of the International Classification of Headache Disorders, primary headaches can be categorized as chronic or episodic; chronic migraine is defined as headaches in the absence of medication overuse, occurring on ≥15 days per month for >3 months, of which headaches on ≥8 days must fulfill the criteria for migraine without aura. Prevalence and incidence data for chronic migraine are still uncertain, owing to the heterogeneous definitions used to identify the condition in population-based studies over the past two decades. Chronic migraine is severely disabling and difficult to manage, as affected patients experience substantially more-frequent headaches, comorbid pain and affective disorders, and fewer pain-free intervals, than do those with episodic migraine. Data on the treatment of chronic migraine are scarce because most migraine-prevention trials excluded patients who had headaches for ≥15 days per month. Despite this lack of reliable data, a wealth of expert opinion and a few evidence-based treatment options are available for managing chronic migraine. Trial data are available for topiramate and botulinum toxin type A, and expert opinion suggests that conventional preventive therapy for episodic migraine may also be useful. This Review discusses the evolution of our understanding of chronic migraine, including its epidemiology, pathophysiology, clinical characteristics and treatment options.