A number of important proteins and enzymes contain copper ions at their active sites. These copper protein sites are associated with a variety of vital biological functions, including oxygen transport and activation, electron transfer, iron metabolism, and superoxide dismutation. The protein ligand imposes an unusual geometric and electronic structure at the copper site. As a consequence, copper proteins often exhibit unique and characteristic spectral features compared to small molecule copper complexes. Studying these spectral features provides insight into the correlation between active site electronic structure and biological function.