 The Global Burden of Disease study has found that headache disorders are among the most prevalent and disabling conditions worldwide. This study updates the documentation of headache epidemiological studies, summarizes global prevalence estimates for all headache, migraine, tension type headache, TTH, and headache on greater than or equal to 15 days per month, H15+, and compares them with global burden of disease estimates. It also examines how methodological factors influence prevalence estimates. The study finds that migraine prevalence estimates have increased over the years, while estimates for all headache types vary between different world regions. This article was authored by Lars Jacobstovner, Kanit Hagen, Matthias Linder, and others.