Much of the chemistry of life depends on carbacyclic organic metabolites, from three carbons (cyclopropanes) to eight carbons (cyclooctanes). The predominant aromatic carbacycle in biology is the phenyl ring system, either as a standalone ring or fused in naphthalene and more extensive fused polycyclic scaffolds. The primary route to benzene rings is from carbohydrates through shikimate, chorismate, and prephenate intermediates, with a key late stage 3,3-sigmatropic rearrangement. Two major types of fused carbacyclic systems arise by either poylketonic-S-enzymes intermediates as in tetracycline assembly, or via carbocation-mediated cyclizations to tetracyclic and pentacyclic hopane, lanosterol, and amyrin frameworks.