A unique feature of the vertebrate brain is the ventricular system, a series of connected cavities which are filled with cerebrospinal fluid (CSF) and surrounded by neuroepithelium. While CSF is critical for both adult brain function and embryonic brain development, neither development nor function of the brain ventricular system is fully understood. In this review, we discuss the mystery of why vertebrate brains have ventricles, and whence they originate. The brain ventricular system develops from the lumen of the neural tube, as the neuroepithelium undergoes morphogenesis. The molecular mechanisms underlying this ontogeny are described. We discuss possible functions of both adult and embryonic brain ventricles, as well as major brain defects that are associated with CSF and brain ventricular abnormalities. We conclude that vertebrates have taken advantage of their neural tube to form the essential brain ventricular system.