Electron paramagnetic resonance spectroscopy (EPR) is a powerful tool for investigating paramagnetic species, including organic radicals, inorganic radicals, and triplet states. The basic principles behind EPR are very similar to the more ubiquitous nuclear magnetic resonance spectroscopy (NMR), except that EPR focuses on the interaction of an external magnetic field with the unpaired electron(s) in a molecule, rather than the nuclei of individual atoms. EPR has been used to investigate kinetics, mechanisms, and structures of paramagnetic species and along with general chemistry and physics, has applications in biochemistry, polymer science, and geosciences.

The degeneracy of the electron spin states is lifted when an unpaired electron is placed in a magnetic field, creating two spin states, m_{s} = ± ½, where m_{s} = - ½, the lower energy state, is aligned with the magnetic field. The spin state on the electron can flip when electromagnetic radiation is applied. In the case of electron spin transitions, this corresponds to radiation in the microwave range.

The energy difference between the two spin states is given by the equation

∆ E = E_{+} - E_{-} = h*v* = *gß*B

where h is Planck’s constant (6.626 x 10^{-34} J s^{-1}), v is the frequency of radiation, ß is the Bohr magneton (9.274 x 10^{-24} J T^{-1}), B is the strength of the magnetic field in Tesla, and g is known as the g-factor. The g-factor is a unitless measurement of the intrinsic magnetic moment of the electron, and its value for a free electron is 2.0023. The value of g can vary, however, and can be calculated by rearrangement of the above equation, i.e.,

*g* = h*v* / ßB

using the magnetic field and the frequency of the spectrometer. Since h, *v*, and *ß* should not change during an experiment, g values decrease as B increases. The concept of g can be roughly equated to that of chemical shift in NMR.