Tetrapyrroles are a class of chemical compounds that contain four pyrrole or pyrrole-like rings. The pyrrole units are linked by =CH− or −CH2− bridges, in either a linear or a cyclic fashion. Pyrroles are a five-atom ring with four carbon atoms and one nitrogen atom. Tetrapyrroles are common cofactors in biochemistry, and their biosynthesis and degradation feature prominently in the chemistry of life.