Originally published in 1979, this book discusses how the physical and chemical properties of disordered systems such as liquids, glasses, alloys, amorphous semiconductors, polymer solutions and magnetic materials can be explained by theories based on a variety of mathematical models, including random assemblies of hard spheres, tetrahedrally-bonded networks and lattices of 'spins'. The text describes these models and the various mathematical theories by which the observable properties are derived. Techniques and concepts such as the mean field and coherent approximations, graphical summation, percolation, scaling and the renormalisation group are explained and applied. This book will be of value to anyone with an interest in theoretical and experimental physics.