Low band gap (E(g) < 1.5 eV) polycyclic hydrocarbons have become one of the most important types of materials for many applications, for example, as semiconductors in organic field effect transistors (OFETs), as light-harvesting dyes in organic solar cells and photodetectors, as near infrared (NIR) fluorescent probes in high resolution bio-imaging and bio-sensing, and as chromophores in non-linear optics. The benzenoid polycyclic hydrocarbons as nano-sized graphene fragments also serve as perfect model compounds to understand the fundamental structure-property relationship of graphene. The ground state of these molecules can be described as either a closed-shell or an open-shell structure on the basis of their molecular size and edge structure. In this review, a summary will be given on a series of low band gap polycyclic hydrocarbons about their synthesis, physical properties and material applications.