This set of essays was given as lectures at the 4th Waterloo International Summer School on Nuclear Magnetic Resonance held in June 1975 at the University of Water loo. The school was sponsored by the National Research Council of Canada and by the Canadian Association of Physicists. These Contributions are introductory and were not intended to be review papers. For valuable help, I would like to thank R. S. Hallsworth, D. W. Nicoll, and R. T. Thompson. M.M.Pintar Table of Contents A Guide to Relaxation Theory. By A. G. Redfield .................... 1 Thermodynamics of Spin Systems in Solids....