It is amazing how much information can be gleaned from a magnetic resonance spectrum by one who knows. That series of lines on chart paper may conceal anything from energies of activation and spin densities, to conformations and differentiation of isomers. In order to be able to deduce such things about the structure and properties of molecules in a sample, it is necessary to be familiar with the underlying principles, and to arrive at that state of understanding is not easy. This book was conceived and written in an attempt to clarify what is necessary theoretical equipment for anyone wishing to extract the maximum information from a magnetic resonance spectrum. It is also written for those who will find a fascination and great satisfaction in the way this subject, which involves so many sides of modern physics, holds together. It seems to the author, from experience, that the difficulty of getting to grips with the theory of magnetism and magnetic resonance is two- sided. On the one hand one has forgotten, or never really known, the principles of electromagnetism on which it is based, and on the other, detailed analysis of the spectra requires a certain facility with the operator techniques of quantum mechanics. In both cases the principle difficulty appears to be unfamiliarity, so the chief aim in this book will be to introduce the enquirer to the relevent language in a reasonably connected fashion.