A detailed picture of the unique electronic structure of active sites in copper proteins is essential to our understanding of their biological functions. This has been achieved through a combination of complementary spectroscopic techniques, chemical perturbations, and parallel studies on copper complexes which serve as spectral analogues. These studies provide a basis for the generation of a "spectroscopically effective" picture of the active site and allow a correlation of changes in geometric and electronic structure with variation in function. When combined with high-resolution crystal-lographic information, single crystal spectral studies allow a correlation of electronic features with specific geometric features and provide a great deal of insight into the bonding at the site...