Nuclear magnetic resonance (NMR) is a form of radio frequency spectroscopy based on interactions between nuclear magnetic dipole or electric quadrupole moments and an applied magnetic field or electric-field gradient. This article provides an overview of the fundamental principles of nuclear magnetic resonance with emphasis on nuclei properties, the basic equation of nuclear magnetic resonance, the classical theory of nuclear magnetization, line broadening, and measurement sensitivity. It describes the pulse-echo method for observing NMR. The article provides useful information on ferromagnetic nuclear resonance and nuclear quadrupole resonance, and illustrates the experimental arrangement of NMR with a block diagram. It also presents several application examples.