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. An Elementary Introduction. By J. Jeener. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 . . . . . . . . . . . . Coherent Averaging and Double Resonance in Solids. By J. S. Waugh. . . . . .. . 23 Macroscopic Dipole Coherence Phenomena. By. E. L. Hahn . . . . . . . . . .. . . 31 . Nuclear Spins and Non Resonant Electromagnetic Phenomena. By G. J. Bene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . 45 . . . . . . . . . Nuclear Spin Relaxation in Molecular Hydrogen. By F. R. McCourt. . . . . . .. . 55 Longitudinal Nuclear Spin Relaxation Time Measurements in Molecular Gases. By R. L. Armstrong . . . . . . . . . . . . . . . . . . . . . . . . .. . . . 71 . . . . . . . Spin-Lattice Relaxation in Nematic Liquid Crystals Via the Modulation of the Intramolecular Dipolar Interactions by Order Fluctuations. By R. Blinc . . .. 97 NMR Studies of Molecular Tunnelling. By S. Clough . . . . . . . . . . . . .. . . . 113 . . Effect of Molecular Tunnelling on NMR Absorption and Relaxation in Solids. By M. M. Pintar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 125 . . . . . . . . . .