HF pulsed ENDOR is generally applied to biological and related model systems. Applications have been primarily to biology with a heavy focus on photosynthesis related radicals or paramagnetic metal ions centers in matalloenzymes or metalloproteins. Additional applications have been to magnetic resonance imaging contrast agents. HF ENDOR has been used as a characterization tool for porous materials, for the electronic properties of donors/acceptors in semiconductors, and for electronic properties of endohedral fullerenes. Framework Substitution with W-band ENDOR has been used to provide experimental evidence that a metal ion is located in the tetrahedral framework and not in a cation exchange position. Incorporation of transition metal complexes into the framework of molecular sieves is of consequence as it could lead to the development of new materials with catalytic properties. ENDOR as applied to trapped radicals has been used to study NO with metal ions in coordination chemistry, catalysis and biochemistry.