Pericyclic reactions have long been considered a biosynthetic rarity. In contrast to their prominence in the laboratory, they seemed to play only minor roles in Nature. Over the past decades, however, enough examples have been amassed to demonstrate that pericyclic reactions occur quite frequently in biosynthetic pathways. Pericyclic reactions include cycloadditions, sigmatropic rearrangements, and electrocyclizations. Biosynthetic cycloadditions have been recently featured in a comprehensive review by Williams.1 The aim of the present article is to provide a counterpart highlighting electrocyclic reactions that have been found or suspected in the biosynthesis of natural products. In many cases, these biosynthetic considerations have inspired biomimetic syntheses, whose success in turn validates the proposal. We consider reactions as biosynthetic if they occur in a given organism or in its immediate environment. This definition includes reactions that occur spontaneously and do not require enzyme catalysis, which are very common among electrocyclizations. Biosynthetic and biomimetic electrocyclizations have never been systematically reviewed. Since numerous examples have recently surfaced in the literature, we feel that the time has come to make such an attempt. For the sake of simplicity, we use the term “electrocyclizations” both for the forward reaction and electrocyclic ring openings. This review is organized primarily along the number of electrons involved in a given π system that undergoes the electrocyclic reaction. Following electrocyclizations of 4π systems, electrocyclic reactions involving 6 and 8 π-electrons are discussed. Thermal and photochemical reactions are presented in that order. Systems including heteroatoms (usually oxygen but occasionally nitrogen), which are confined to 6 π-electrons, are presented separately. Finally, the question of whether biosynthetic electrocyclizations require enzyme catalysis is briefly addressed. One of the most attractive features of biomimetic electrocyclizations is that they often participate in pericyclic reaction cascades.2 In combination with cycloadditions, for instance, they are able to rapidly generate molecular complexity and diversity, an aspect that is given ample attention in this review. If the cascade involves several electrocyclizations, priority is given to the highest number of π-electrons. In some cases, the occurrence of electrocyclic reactions or reaction cascades in biosynthetic pathways is quite speculative. Nevertheless, these cases have been included provided they have inspired biomimetic syntheses.2 By contrast, total syntheses featuring electrocyclic reactions that have not been proposed or are unlikely to be biomimetic are generally not covered in this review. This means that many elegant synthetic applications of electrocyclizations, such as Woodward’s celebrated porphyrin to chlorin conversion used in the synthesis of chlorophyll, will not be discussed.3 The theory of electrocyclizations will not be recapitulated here either. The reader is referred to a series of reviews that have appeared on this subject.4 As a reminder, the stereochemical course of electrocyclizations in the ground state and excited state is summarized below: