The core of this little book is a survey of the experimental work carried out by our research group in the course of two decades. Although the writer has made an effort to present and evaluate related contributions of many authors, the text should not be considered as a monograph of encyclopedic character. Our experiments began with a study of cis-Irans-isomeric C - 40 carotenoids and were subsequently extended to other types of polyenes in order to determine how far the observations were applicable to broader fields. Similar considerations have led to the inclusion of the Chapters on Vitamins A and on Cumulenes although we have not contributed to the stereochemistry of these classes of compounds. The first eleven Chapters have been published in Volume XVIII of the Series, Progress in the Chemistry of Organic Natural Products and now appear in a revised, up-to-date form. Sincere thanks are due to the following former members of our research group who have participated in the experimental elucidition of the field: W. V. Bush, A. Chatterjee, L. Cholnoky, J. Dale, R. B. Escue, P. Fischer Jorgensen, Ch. Gansser, F. T. Haxo, G. Karmakar, B. K. Koe, J. Leemann, R. M. Lemmon, the late A. L. LeRosen, W. Lijinsky, K. Lunde, E. F. Magoon, W. H. McNeely, F. J. Petracek, J. H. Pinckard, A. Polgar, A. Sandoval, W. A. Schroeder, J. W. Sease, Y. W. Tang, K. Tsukida, the late P. Tuzson, L. Wallcave, and B. Wille. Dr.