In order to comprise all headache patients, we have also included epidemiologic studies that have investigated headache in general. The term “headache” is not, however, defined in the ICHD classifications, and we have therefore included studies on headache prevalence that appeared before 1988. For TTH, the term “chronic” has been applied to patients who have this type of headache for ≥15 days per month on average for ≥3 months (ICHD-1 and -2). In many headache studies, a similar definition has been given to patients with headache, irrespective of whether it is of the tension type or not. We have also gathered data on “chronic headache” (i.e. ≥15 days per month or “daily” headache) to assess the prevalence of these patients who are probably most incapacitated by their condition. A subgroup of these patients are overusing acute medication, a condition termed “Medication overuse headache” (MOH) in the IDHD-2. We have also collected data on this frequent and possibly preventable condition.