The headache literature is characterized by controversy regarding theoretical explanations, pain mediating processes and treatment efficacy. The present paper reviews past assessment and classification practices within this literature. The data suggest that inadequate adherence to the iagnostic scheme originally proposed by the Ad Hoc Committee on Classification of Headache has resulted in major nosological errors. Thus, the literature presents a confusing array of inconsistent results and inconclusive investigations. Criteria for classification of head pain are provided and the utilization of more sophisticated assessment methodology advocated. It is hypothesized that an idiosyncratic analysis of head pain parameters will eventuate in the empirical delineation of well-defined subgroup populations of migraine, muscle-contraction, psychogenic, and combined headache patients.