This thesis compirses three parts, Parts A, B and C. Each part is divided into a number of principal sections prefixed by Roman numerals, and these sections are further divided into sub-sections prefixed by Arabic numerals. Each sub-section is uniquely indicated by a combination consisting of a capital letter, a Roman and Arabic numeral, e.g., (A, IV, 1) – Perinaphthene System: Nomenclature. Part A commences with a brief analytical survey based on the chemical literature of the various aromatic carbocyclic systems and this is developed logically into a consideration of the perinaphthene system. A detailed review of the latter and its simple derivatives follow, with particular emphasis on the theoretical aspects of this system. Part B is an account of the results achieved by the author in the course of investigations centred on aromatic compounds containing the perinaphthene nucleus. The synthesis and properties of indene [2,1-a] perinaphthene are described, in addition to preliminary experiments with a view to preparing cyclopenta[a]-perinaphthene. The outcome of the investigation is fully examined from the theoretical standpoint. Part C is entirely devoted to experimental details, and is the complement to Part B. Ten plates, mainly of absorption spectra are to be found at the end of the thesis.