As traced by Uichael Bradbury in his recently published monograph, The Concept of a BZood-Brain Barrier, the idea of a scientific challenge is just about as old as the twentieth century. Curiously, it remains undefined. Is it a structure or structures as some use the term, or is it a reciprocal per­ meability, a force-flow relationship, as do others, or is it a group of processes, some more specialized than others? Depend­ ing upon the observer, the method, and what is observed, it seems to be each of these or all. This Symposium takes as its focus of interest the micro­ vasculature of the brain and includes considerations of blood flow, the properties of vessel walls and the control of flow and permeability. In addition perturbations that change the characteristics of the flow of materials are given attention. By changing the usual focus of interest, the organizers, Drs. Suddith and Eisenberg, have given a fresh outlook to the subject and now, by publication of the Proceedings, have arranged for wide availability of these interesting papers. Keasley Welch v PREFACE A symposium on the cerebral microvasculature and its function in the blood-brain barrier was held at The University of Texas Medical Branch, Galveston, in the summer of 1979. Investigators from the United States and Europe met to discuss their recent work.