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 diagnostic 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. (Headache 22:221-232, 1982) Headache is currently one of the most active areas of scientific investigation. Clinical, experimental, and theoretical activity has prolifereated in attempts to understand the pain mediating processes of head pain as well as select optimal treatment interventions. A number of recent reviews have appeared, 1-10 but contradictory conclusions have been reached. 11,12 For example, in regard to treatment strategy, some investigators equate the efficacy of biofeedback and relaxation training for "tension headache," 4,9 while others do not. 10,11 In addition, investigators have also reached different conclusions regarding the effectiveness of blood volume pulse feedback training for migraine. 2,4 Furthermore, disagreement about physiological mechanisms of headache exist as well. For example, the traditional view of "tension headache" has been attacked 8 and disagreement concerning pain mechanisms for migraine are also evident. 2,13 Thus, the recent proliferation of headache research has failed to produce consistent findings and conclusions.There is agreement among reviewers that several methodological issues may account for some of the discrepant data noted above. These have included subject variables, procedural variables, outcome measures, questionable data interpretations, and many other problems. While these critiques are certainly valid, a primary methodological concern -assessment -has not received adequate attention. As mentioned previously, the headache literature is primarily concerned with the pain mediating processes of headache and selection of optimal treatment strategies. However, before one can answer these questions, the accurate identification of the clinical phenomenon of interest is necessary. In other words, operationalization of criteria and appropriate classification of headache types must precede attempts to identify pain mediating processes and optimal treatment modalities. Furthermore, accurate utilization of diagnostic and nosological criteria is essential for any investigation of headache. An analysis of these past assessment practices may help explain the origin of some of the current confu...