Zinc is nutritionally essential and indispensable to growth, development, and maintenance of human functions. There is hardly any cellular process that does not depend on zinc in some way. As a constituent of at least 2800 human proteins and with cellular concentrations of a few hundred micromolar, zinc has unparalleled significance in protein structure, enzymatic catalysis, and cellular regulation. The largest group of zinc metalloenzymes are proteinases. Zinc has a major role in the structural organization of protein domains that interact with DNA/RNA, other proteins, and lipids. Several dozen proteins control cellular and subcellular zinc homeostasis and re-distribution. The control is a prerequisite for regulatory functions of free zinc (II) ions. Fluctuations of cellular free zinc ion concentrations modulate the biological activity of a yet unknown number of additional proteins, suggesting roles of zinc in information transfer.