This Ciba Symposium records the proceedings of a meeting held in London in May, 1957 to discuss the cerebrospinal fluid. The papers presented may be roughly grouped in the fields of anatomy, physiology, and clinical medicine. Those in the field of anatomy are excellent and in keeping with the current renaissance in this field. They include observations on the early development of the telencephalic choroid plexus, suggesting a progression of diverse functions for this tissue during ontogeny, and a study of the mature choroid plexus using the electron microscope.