Fractals and disordered systems have recently become the focus of intense interest in research circles. This book discusses in detail the effects of disorder on mesoscopic scales (fractures, aggregates, colloids, surfaces and interfaces, glasses and polymers) and presents tools to describe them in mathematical language. A substantial part of the book is devoted to the development of scaling theories based on fractal concepts. In 10 chapters written by specialists in the field, including E. Stanley and B. Mandelbrot, the reader is introduced to basic concepts and techniques in disordered systems and is to current research. In each chapter the connection between theory and experiment is emphasized, and a special chapter entitled "Fractals and Experiments" presents experimental studies of fractal systems in the laboratory. It can be used as a textbook for graduate students, by university teachers to prepare courses and seminars and by active scientists who want to become familiar with this new field.