Analyses of zinc protease sites using X-ray diffraction, NMR and X-ray absorption fine structure in combination with functional studies of these enzymes are beginning to reveal the manner in which the protein modulates the properties of the zinc to achieve specificity and catalytic efficiency. The chemistry of zinc: 1) permits a variety of types and numbers of ligands and coordination geometries, 2) promotes ionization of water at neutral pH values and 3) is inert to oxido-reductants present in the medium. The type of ligands and the protein scaffolding of the zinc binding site determine the binding strength of zinc and its catalytic properties. These properties in turn are critical to the role of zinc in a wide variety of metalloexo- and endo-protease-catalyzed processes.