This book provides a complete exposition of the theory of nuclear magnetic relaxation caused by the thermal motion of the molecule containing the relaxing nucleus. The author begins by defining the physical quantities encountered in nuclear magnetic resonance studies and surveying pioneering investigations in the field. Nuclear magnetic relaxation by scalar, dipolar, quadrupolar and spin-rotational interactions and by anisotropic chemical shift are then examined in detail. Relaxation rates are expressed in terms of spectral densities, and the values of the spectral densities for various molecular shapes are calculated by random walk or Brownian motion dynamics. The text should be within the grasp of readers who have taken undergraduate courses in electromagnetic theory and in classical and quantum mechanics, although topics in these fields of particular relevance are to be found in appendices. This book will be of value to postgraduate students and research workers using n.m.r. in physics and physical chemistry departments, and by scientists in industrial and medical research.