ENDOR technique has been used to characterize of spatial and electronic structure of metal-containing sites. paramagnetic metal ions/complexes introduced for catalysis; metal clusters producing magnetic materials; trapped radicals introduced as probes for disclosing the surface acid/base properties; color centers and defects as in ultramarine blue and other gems; and catalytically formed trapped reaction intermediates that detail the mechanism. The application of pulsed ENDOR to solid samples provides for many advantages compared to CW ENDOR. Such advantages are the generation of distortion-less line shapes, manipulation of spins through a variety of pulse sequences, and the lack of dependence on a sensitive balance between electron and nuclear spin relaxation rates and applied power (given long enough relaxation rates).