Even in crystalline matter most of the interesting phenomena are caused by imperfections in the structure. When the disorder is radical, as in disordered alloys or - even more - in liquids, glasses, quasi-crystals and amorphous solids, physics encounters different phenomena and new problems, many still unsolved. This book comprehensively introduces current research into disordered matter, liquid and solid, metallic, insulating and semiconducting. Its approach is to describe pertinent properties and the basic theoretical methods for explaining them, and then to exemplify the directions of current research. It is intended for postgraduate students, advanced undergraduates and research workers in related areas. Basic notions of order and disorder are introduced first, followed by a discussion of the related problems of how to describe disordered structures and how to find out experimentally what they are. In this context the measurement by x-ray and neutron scattering of the various kinds of structure factor are discussed. Fractal descriptions are introduced. The importance of computer simulation studies is emphasised throughout, for example in the extensive chapter on the theory of liquids and also as a complement to neutron inelastic scattering in the study of thermal motion in liquids and glasses. Recent achievements in applying the nearly-free-electron model to electron transport and thermodynamic behaviour in disordered metals are explained and, when this approximation fails, the Green function, coherent potential and other techniques are shown to be the current methods of choice for electronic properties. A chapter is devoted to percolation and Anderson localisation and their relevance to many different phenomena such as conduction by hopping. All these concepts are brought to bear on liquid and glassy metals, amorphous semiconductors, simple liquids and non-conducting glasses, expanded liquid metals and compound-forming liquid alloys. At all points experimental results are presented to test the theories and many directions of current experimental development are pointed out