Obtaining accurate and reliable information on the prevalence of migraine is essential to understanding the burden it places on society. Although the epidemiology of headache has been described in more than 50 population-based studies, only 24 of these have described the gender- and age-specific prevalence of migraine. Essentially five different case definitions have been used in these studies. Variation in the prevalence of migraine among studies is largely due to differences in case definition and in the age and gender distribution of study populations. Among four recent studies that used the diagnostic criteria of the International Headache Society (IHS), a coherent picture emerges. The prevalence of migraine is approximately 6% among men and 15 to 17% among women. Prevalence varies by age, increasing to about age 40 years and declining thereafter in both men and women. The gender ratio also appears to vary by age, increasing from menarche to about age 42 years and declining thereafter. Although the use of the IHS criteria has resulted in a more coherent picture across population-based studies, efforts must be made to assess the reliability and validity of these criteria in population-based samples.