Brain Barrier Systems, Volume 29In recent years, many important developments have occurred in many research disciplines that are directly related to brain permeability. In morphological fields, the fine resolution provided by electron microscopy has resulted in new concepts of membrane structure. Electron microscopy, combined with autoradiography, have contributed significant information concerning dynamic events occurring in and around such membranes. Permeability measurements with drugs have led to new theories of the physical and chemical requirements which determine the penetration of drugs