In 1972 Erich Clar formulated his aromatic π-sextet rule that allows discussing qualitatively the aromatic character of benzenoid species. Now, 40 years later, Clar's aromatic π-sextet rule is still a source of inspiration for many chemists. This simple rule has been validated both experimentally and theoretically. In this review, we select some particular examples to highlight the achievement of Clar's aromatic π-sextet rule in many situations and we discuss two recent successful cases of its application

The following organizations are thanked for financial support: the Ministerio de Ciencia e Innovación (MICINN, project number CTQ2011-23156/BQU), the Generalitat de Catalunya (project number 2009SGR637, Xarxa de Referència en Química Teòrica i Computacional, and ICREA Academia 2009 prize), and the FEDER fund (European Fund for Regional Development) for the grant UNGI08-4E-003