About 4% of the adult population and about 1% to 2% of children experience chronic attacks on a daily or near daily basis. While there is uncertainty about the biological mechanisms that lead to headache "chronification," the epidemiologic literature has provided some insight into modifiable and nonmodifiable factors that appear to influence risk of headache progression. This review summarizes the evidence from population-based studies related to the chronic daily headache phenotype, natural history, and risk factors that may influence incidence, prevalence, or prognosis.