This book attempts to combine many different threads into a comprehensible whole. Since the subject is the Gamma Knife and the author is a neurosurgeon, the field of clinical interest is restricted to intracranial pathology. The discipline of radiosurgery now applies to patients who may reasonably be referred by internists, neurologists, otolaryngologists, endocrinologists and several others. Some of the topics, touched upon, such as stereotaxy and the construction of a radio surgical instrument are unfamiliar to the majority of medical men. Other topics, such as those pertaining to the reactions between radiation and living tissue, are not exactly unfamiliar and yet, for most of us, they are not comfortable areas of expertise: in that we have some basic knowledge but not enough to draw conclusions and interpret. In particular, it is not easy to answer the very sensible questions that patients ask, when being considered for this particular form of treatment. The author has attempted to describe the basic relevant phenomenology in terms that should be readily understandable to a non-specialist physician. To do this, he has been heavily dependent on the expertise of a number of mathematically sophisticated collaborators, who have checked his manuscript. They are named in the acknowledgments section. The relevance of the different sections of this book will naturally be assessed differently, according to the experience and interest of the reader. To simplifY access to the information that is required, the book is divided into three main sections.