Zinc ions are key structural components of a large number of proteins. The binding of zinc stabilizes the folded conformations of domains so that they may facilitate interactions between the proteins and other macromolecules such as DNA. The modular nature of some of these zinc-containing proteins has allowed the rational design of site-specific DNA binding proteins. The ability of zinc to be bound specifically within a range of tetrahedral sites appears to be responsible for the evolution of the wide range of zinc-stabilized structural domains now known to exist. The lack of redox activity for the zinc ion and its binding and exchange kinetics also may be important in the use of zinc for specific functional roles.