This is the last of four chapters that examine the kinds of carbon-based reaction intermediates/transition states that occur in the organic chemistry of life. The chapter raises the issue of stepwise vs. concerted reaction mechanisms, particularly in cascade reaction where multiple C–C bonds are broken and made. It then turns to bona fide reaction categories with no intermediates – pericyclic reactions. Pericyclases are enzymes that catalyze conversion of bound substrate(s) to bound product(s) by concerted mechanisms – no discrete intermediates. Although as yet a small class of enzymes, more pericyclases are turning up as a myriad of fungal genomes are examined for natural product biosynthetic pattens. Four types of pericyclic reactions that occur in metabolic pathways include: sigmatropic rearrangements, electrocyclic rearrangements, Diels–Alder 4 + 2 cyclizations, and Alder-ene rearrangements.