This book is a survey of some aspects of current knowledge on regional Cerebral Blood Flow (r CBF), mainly as studied by the isotope clearance method. Although both theoretical and methodological problems are discussed, attention is mainly dedicated to data obtained from clinical studies. The papers which make up this book were presented at the International Symposium on the Clinical Applications of Isotope Clearance Measurement of Cerebral Blood Flow, held in Mainz, Western Germany, on April 10-12, 1969. The previous meetings on Cerebral Blood Flow, held in Lund in 1964* and in Lund and Copenhagen 1968**, had shown that the moment had come to concentrate on the possibilities of introducing r CBF measurements into clinical routine. This is why in the Mainz Symposium attention was initially focused on methodological aspects. This is also why theoretical problems of physiology of CBF were not emphasized. Finally, this explains why such topics as cerebrovascular disease, head trauma, coma, carotid surgery, brain tumors and intracranial pressure were given pride of place. However a survey of the clinical aspects of r CBF measurements would not be complete without an account of the application of such measurements to monitor cerebral circulatory changes during anesthesia and therapeutic procedures like, for instance, hyperventilation and hyperbaric treatment. Furthermore, it is now possible to obtain data from correlative r CBF studies per- formed before, during and after surgical operations on the human brain.