Combined quantum mechanical and molecular mechanical methods (QM/MM) are one of the most promising approaches for quantum mechanical calculations of chemical processes in solution and in enzymes. In such a method a relatively small part of the system (e.g. the solute) is analysed through quantum mechanics and the remainder (e.g. the solvent) is represented through molecular mechanics, thus combining the accuracy of one method with the efficiency of the other. This book provides an in-depth survey of the methods and their applications in chemistry and biochemistry.