This book is an attempt to fill a gap in the existing literature on ion exchange. The many excellent works already available are of three main types, general introductions to the subject, specialist discussions of analytical and laboratory techniques, and advanced theoretical treatises. In practice, in spite of the vast number of processes which have been developed for la bora tory use, 99 per cent of all ion exchange resins produced in the world are used in water treatment, or closely allied applications. This book is intended as a general survey of the principles governing the practical uses of ion exchange resins, for the benefit of students encountering the subject for the first time, and for the chemists and engineers in many branches of industry whose work brings them into contact with water treatment, but who do not have the time to study more advanced volumes of basic theory. The background presented has been simplified to the maximum extent found possible without falsification, and an attempt has been made to relate each aspect of theory to its practical consequences in full scale water treatment. Mathematical methods have been avoided and pictorial or graphical presentation methods used wherever possible. As the book is concerned with general principles, rather than details of any particular research work, references to original papers and patents have been omitted except in the cases of special processes, which have a single clearly defined origin.