Tetrapyrroles are intensely colored natural products of vital importance in the biosphere for essential processes such as respiration and photosynthesis and are also of key importance as cofactors in a number of other enzyme reactions. Tetrapyrroles may either be linear in nature, as found in the bilins, or cyclic as in the hemes, chlorophylls, and corrins. In the cyclic tetrapyrrole group, the four centrally located pyrrole nitrogen atoms of the macrocyclic ring offer a range of possibilities for metal chelation. Modulation of the properties of the metal-lotetrapyrrole prosthetic groups by individual proteins give rise to a remarkably versatile family of powerful bio-organic reagents.