Polycyclic hydrocarbons are of interest in many fields of science: theoretical chemistry, physical chemistry, organic chemistry, dyestuff chemistry and biology. With regard to the latter, I am indebted to Dr. Regina Schoental of the Medical Research Council for the review in this present work of carcinogenesis by polycyclic hydrocarbons. This book is designed to present the facts in a simple and clear order and to derive empirical rules from them, but it does not present a com- prehensive theory about polycyclic hydrocarbons. An attempt is made instead to extend classical symbolism into modern structural chemistry. Thus extensive use is made of Robinson's aromatic sextet, which is applied in an uncompromising and strict way. This quasi-classical attempt is encouraged further by such completely unexpected dis- coveries as those of Dewar benzene and of the electronic asymmetry of formally symmetric hydrocarbons. How difficult it is to break away from any established way of thinking has been admirably expressed by Kekule ( Organische Chemie , 1861, Part 1, page 4, translated from the German): All our ideas are based, to an extent much greater than we ordinarily believe, on those of our predecessors. Our accumulated experience, the notions of which our training has accustomed us to, of whatever kind they have been, influence the course of our thoughts far more than we are willing to admit; only too frequently the following of our regularly used, well trodden way of thinking leads us to overlook the simplest of correlations.