This volume represents the formal presentations and discussions which took place during a three-day meeting in March 1988 at The University of Texas M. D. Anderson Cancer Center in Houston. It is" dedicated to my friend of more than thirty years, Prof. Dr. Klaus Joachim Ziilch, who died in Berlin on December 2. 1988 while this volume was still in preparation. Klaus Zulch had devoted a significant portion of his professional life to a better understanding of central nervous tumors. Over the past two decades he served as the Director of the Collaborating Center for CNS Tumors, under the auspices of the World Health Organization (WHO), and it was largely through his efforts that the work of the CelJ. ter in developing criteria for a histologic classification of these neoplasms was kept alive. Without his stimulus this Houston meeting would probably not have taken place. In early 1987 he approached me with the idea of convening, at an early date, a meeting in Houston in collaboration with the Department of Neuro-Oncology of the Cancer Center, of which I was then Chairman. The purpose of this proposed meeting was to discuss recent research developments that might have a profound influence on the classification of brain tumors and ultimately necessitate revision of the "Blue Book" of the WHO on Histological Typing 0/ Tumours 0/ the Central Nervous System.