Geometric group theory studies groups as realized as symmetries of metric spaces. One of the most important classes of groups are hyperbolic groups', the subject of this book. They have a beautiful and robust theory, which is explored from the beginning of the theory right up to the forefront of current research. It will suitable for an advanced graduate class, or for study by those beginning in the field provide a reference for experts and outsiders alike. The book starts from the beginning (at a level appropriate for graduate students just beginning in the field) and works up to somewhere near the current research in the area. The book also provides a valuable reference for experts, as well as mathematicians in other areas hoping to learn something about the field.