High-altitude headache (HAH) is an important public health problem because many of the millions of visitors to locations high above sea level get significant headaches each year. Headache is the most common symptom of acute exposure to high altitude. It may be a manifestation of acute mountain sickness (AMS), as well as of chronic mountain sickness (CMS). This article describes the clinical picture of AMS and CMS. The clinical characteristics of HAH are presented, its pathophysiology is discussed, and the acute and preventive treatment options are reviewed.