From the time dependence of the nuclear spin relaxation phenomena, molecular structure, conformation, motions, interactions, diffusion, and adsorption can readily be studied. Aside from the studies at the molecular level, nuclear spin relaxation phenomena have been applied to qualitative and quantitative analysis in the areas of polymer research, agriculture and food research, petroleum research, and medical research. This article presents an introduction to the theory, techniques, and applications of nuclear spin spectroscopy.