An ideal introduction to this fast growing area, this Primer describes the many computational methods currently used by practising chemists. The authors describe the various techniques available, and how they can be applied to single molecules, to assemblies of molecules, and to molecules undergoing reaction. An introductory chapter outlines hardware and software options, as well as investigating some applications and developments. Subsequent chapters cover quantum mechanics, molecular mechanics, statistical mechanics, the modelling of biomolecules, and drug design.