With the advent of computed tomography (CT) scan, much information has been obtained about the pathophysiology of hydrocephalus. It is now clear that hydrocephalus is not a disease entity but rather a syndrome or sign resulting from disturbances in the dynamics of cerebralspinal fluid (CSF) caused by various diseases. Consequently, it has become necessary to revise its definition and classification. In this paper, a contemporary definition and classification of hydrocephalus are presented. Also, a classification of "intractable hydrocephalus"--with its diagnostic criteria--which is a clinically unsolved problem, is attempted, bearing in mind its place in the clinical management and future investigation of the pathophysiology of hydrocephalus.