Nuclear magnetic resonance (NMR) is the magnetic resonance of an atomic nucleus. All atomic nuclei, except even-even nuclei (those with an even number or protons and an even number of neutrons), have magnetic moments associated with them, which tend to be aligned by an externally applied magnetic field, but because nuclei possess angular momentum, they precess about the direction of the applied field. The energy of the interaction between the applied and the nuclear magnetic fields is quantized, so that only certain orientations for the nucleus relative to the the applied field are permitted: a transition from one orientation to another involves the absorption of emission of a quantum of electromagnetic radiation, the frequency of which can be shown to equal the precessional frequency.

With the magnetic field strengths customarily used the energies involved are small, and the radiations fall in the radio frequency band (1–100 MHz). Transitions from one energy level to another can be induced by applying a second magnetic field, at right angles to the first, which rotates in phase with the nuclear precession. NMR spectroscopy consists of observing the point of resonance at which such transitions are induced. Data obtained in this way are provide valuable information concerning nuclear properties. As the orbital elements shield the nucleus to a certain extent from the applied magnetic field, at a given frequency nuclei in different electronic (i.e. chemical) environments will resonate at slightly different values of the applied field. This phenomenon, known as the chemical shift, enables NMR spectroscopy to be of great value in working out the configuration of complex molecules. Nuclear magnetic resonance also forms the basis of magnetic resonance imaging (MRI)