The physical principles on which downhole NMR is based are complex. The magnetic resonance phenomena and the pulse sequences, for example, are related to quantum mechanics. NMR logging measurements and NMR log interpretation, however, may be understood with the knowledge of a few basic NMR concepts. These concepts include nuclear magnetism, polarization, T1 relaxation time, T2 relaxation time, pulse tipping, free induction decay, spin echoes, and CPMG and inverse-recovery pulse sequences for one-, two-, and three-dimensional NMR measurements. This chapter is devoted to a discussion of these concepts. More complete and advanced discussions of the basics of NMR physics are found in many fine textbooks referenced at the end of the book.