Originally published in 1969, this book analyses the special properties of benzene and its derivatives, which give them their distinct aromatic character. The text first discusses the structure of benzene, including the light shed on it by physical methods and wave mechanics, and the structure of polycyclic benzenoid hydrocarbons and heterocyclic aromatic compounds. Aromaticity is defined as a function of the electronic structure of the molecule or ion in question and it is shown that this has important consequences in respect of bond lengths, resonance energies, electronic absorption spectra and induced ring currents. Final chapters discuss how far aromatic character and aromaticity are associated with non-benzenoid hydrocarbons and with more complex systems.