• chronic daily headache
• primary chronic daily headache
• secondary chronic daily headache

Chronic daily headache (CDH) describes a group of patients whose problem is self evident—they have a lot of headache—but whose management can be among the most challenging that neurologists face. We find it useful to employ the term in its literal sense, much as a haematologist can make a diagnosis of anaemia without implying any of the myriad of causes. In that analogy the haematologist goes on to seek the underlying pathophysiological construct, and so neurologists might take the same view of CDH. This approach does not imply any necessary pathophysiological link between the various causes of CDH, but allows a practical, and we hope useful, approach to the problem. A simple definition of CDH is headache on 15 days or more a month. This dissects out the disabled group¹ who will often require neurological attention to be managed adequately. It is likely, based on other estimates of primary headache in similar populations,^2,3 that between 4–5% of the UK population suffer from daily or near daily headache for a significant period of any one year. Recent population based studies show that in the USA⁴ and Spain⁵ nearly 5% of unselected populations have daily headache. The proportion remains similar in the elderly,⁶ and the size of the problem in relation to other common forms of headache is substantial (table 1). Data from the Association of British Neurologists shows that headache is the single most common cause for neurological referral in the UK. Our experience suggests that CDH in its various forms presents the most substantial management challenge in outpatient headache referrals. A special clinical subgroup of CDH—that of new daily persistent headache (NDPH)—is so distinct that it is covered elsewhere in this supplement (see page ii6).