This chapter presents the existing data concerning the epidemiology of selected forms of hydrocephalus, concentrating on congenital and infantile hydrocephalus and idiopathic and secondary normal pressure hydrocephalus (NPH). The epidemiology of congenital and infantile hydrocephalus has been explored in several studies. Casmiro et al. based the diagnosis on absence of known causes of secondary NPH, impaired gait, and CT scans showing findings indicative of NPH. The chapter explores the epidemiology of idiopathic normal pressure hydrocephalus (iNPH) in a Norwegian county of 220000 inhabitants, by actively informing the public and professional health workers about the condition, asking for referral of suspected individuals on a broad clinical basis. The lack of universally accepted guidelines for the diagnosis of iNPH, and the lack of powerful tests to predict shunt success, probably also contribute to the relative low rate of diagnosis, and consequently, of surgery.