This review focuses on the interrelation between the influences of polymeric structure on the optical properties of polythiophene - a versatile polymer for opto-electronic applications. From the application's point of view, the polymer should either be melt-or solution-processible. Functionalization of polythiophenes really helps for the above. On the other hand, the functionalization at the 3 and/or 4 position of the thiophene ring modifies the band gap of the polymer, which in turn, modifies photo absorption, photo- and electro-emission characteristics of polythiophenes. The present work reviews the influence of the optical properties of polythiophene on the different functional groups present in the thiophene ring. © World Scientific Publishing Company.