Significant subsets of military veterans have reported such health problems as headache, joint pain, fatigue, and memory/concentration difficulties subsequent to their participation in the Gulf War. However, the etiology remains controversial. A number of toxins have been implicated as etiologic factors for GW-related health problems; however, exposure levels have been difficult, if not impossible, to document retrospectively. These difficulties with exposure verification have led GW researchers to de-emphasize methods typical of neurotoxicological research examining exposure-symptom relationships and instead focus on epidemiological approaches emphasizing identification of coherent symptom patterns.