A number of patients attending specialty headache centers complain of very frequent, almost continuous headaches, which are usually grouped together under the term chronic daily headache (CDH), a category which does not appear in the International Headache Society (IHS) classification published in 1988. More than 10 years later, this issue is still debated, also in light of the foreseen revised classification. Several terms have been used to define the clinical picture of CDH, and different criteria have been proposed for the diagnosis of these forms. In most cases, CDH appears to evolve from an episodic migraine, but the temporal limits between an episodic and a no-longer episodic form of migraine are questionable. Although some theoretic problems remain unresolved, it seems that the next revision of the IHS classification can no longer ignore the existence of CDH.