NMR is a technique based on the behavior of atomic nuclei in magnetic fields. It is used in medicine for magnetic resonance imaging, and in chemistry/biochemistry for determining the structure of chemical compounds.

The basic theory behind NMR is that protons, which spin and have charge, have a magnetic dipole (a north and south pole, like miniature magnets themselves). When protons are subjected to a magnetic field from a very powerful electromagnet, they reorient in the direction of the magnetic field. The reorientation causes them to release radio waves, which can be detected.

For medical applications, this effect is sufficient, as different tissues have differing water content, and will return different signals. However, for more sophisticated chemical studies, there are several other effects which take place that allow structure determinations to be made. Protons that are very near to each other, or on adjacent atoms, cause resonance effects that create characteristic peaks. With computer modeling and knowing the sequence of a protein, it becomes possible to obtain very clear structures of proteins and other organic molecules in solution.