The term medication overuse headache (MOH) was recently introduced by the International Headache Society to describe daily or nearly daily (chronic) headache that occurs after the regular intake (overuse) of any kind of antiheadache or antimigraine drug. Chronic headache is a growing problem throughout the world. It affects up to 3% to 4% of the general population, and 30% to 50% of such headaches are attributed to overuse of antiheadache drugs. This article reviews the literature on the epidemiology, risk factors, and pathophysiology of MOH. It also summarizes current strategies for treating and preventing headache chronicity.