Nonplanar distortions of tetrapyrroles are prevalent in the hemes of hemoproteins, the pigments of photosynthetic proteins, and cofactor F₄₃₀ of methylreductase. The nonplanarity of these porphyrin cofactors is currently believed to influence factors in the biological activity of the proteins, in part, because the porphyrin deformations are often conserved within functional classes of proteins. The occurrence, classification, and study of nonplanar porphyrins in proteins and synthetic nonplanar porphyrin analogs are reviewed.