BACKGROUND- Chronic daily headache (CDH) is a disorder commonly encountered in clinical practice. Despite its prevalence, CDH has been poorly characterized, and relatively little is known regarding its biology and optimal medical management.REVIEW SUMMARY- We present a review of the data that support the need to develop a new classification system for the CDH syndromes. The clinical and biological characteristics of chronic tension-type headache, transformed migraine, hemicrania continua, and new daily persistent headache with migrainous features are discussed, and recommendations concerning the medical management of these patients are offered.CONCLUSIONS- CDH is a heterogeneous clinical entity with characteristics and subtypes that require further definition and validation. An accurate classification of CDH may assist the clinician in diagnosis and, likely, reduce the incidence of unnecessary and costly ancillary testing. Preliminary evidence suggests that more careful diagnostic discrimination among the subtypes of CDH will assist in directing pharmacologic intervention.