Metal ions play an essential role in the functioning of all biological systems. They interact with charged and polar groups of all biopolymers. The study of structural and functional properties of metal binding proteins is an important area of modern physical and chemical biology. Metal cations and metal binding proteins are involved in all crucial cellular activities. Many pathological conditions are correlated with abnormal metal metabolism. Encyclopedia of Metalloproteins is a key resource that provides basic, accessible, and comprehensible information about this expanding field.