•ESR or Electron Paramagnetic Resonance (EPR) Spectroscopy •Provides information about the electronic and molecular structure of paramagnetic metal centers. Measurement of the spin state, S, the magnitude of hyperfine interactions with metal and ligand nuclei, and the zero-field splitting of half-integer S > 1/2 electronic states, allows a researcher to identify the paramagnetic center, and to potentially identify ligating atoms.