ESR or EPR is a spectroscopy that is used to study the structure and the dynamics of selected systems that have a paramagnetic center.

In most cases the paramagnetism derives only from the angular spin momentum S of unpaired electrons, then the acronym ESR (Electron Spin Resonance) is also encountered.

Paramagnetic centres can be naturally found in many systems but, at request, they can also be introduced on purpose; for this reason EPR is used in many different fields: chemistry, physics, biology, medicine,…

Analysis of the spectra allow to obtain information that often other techiques can not obtain.