Abstract: Conjugated polymers have been studied extensively in the last decade both in the doped conducting state and the undoped semiconducting or insulating form. For the doped conducting polymers, the nature of the metallic state upon doping and the dimensionality of the system have been a hot issue, with improvements in chemical processing playing the key role in clarifying them. For the undoped polymers, particularly the luminescent polymers, the nature of the electronic and optical processes occurring upon charge injection or photoexcitation have been under intense study be¬cause they reflect the underlying physics in conjugated polymer based light-emitting (electroluminescent) devices. In this dissertation, both issues are studied. In the conducting state, systematic charge transport and structural studies were performed on hydrochloric and camphor sulfonic acid doped polyaniline and its methyl derivative poly(o-toluidine) (POT) fibers to investigate the role of chemical processing and the nature of the metallic state in conducting polymers. The study shows that the charge transport properties are controlled by the local microstructural order whose formation is very sensitive to chemical processing conditions such as processing temperature, dopant and solvent used. The solvent, or the so-called secondary dopant, has been shown to play a critical role in improving the charge transport properties. As a result of improved chemical processing, high quality camphor sulfonic acid doped poly(o-toluidine) (POT-CSA) fibers have been prepared from m-cresol, enabling a direct experimental test of the random-dimer model (RDM), one of the models proposed to account for the metallic state in polyaniline. The results of the charge transport studies on the POT-CSA fibers and the dielectric response studies of the POT-CSA solutions in formic acid are clearly inconsistent with the RDM. In the semiconducting/insulating states, pyridine-based fluorescent polymers were used to study the electroluminescent processes and properties. Light-emitting diodes based on these polymers were fabricated and characterized in various device configurations. In addition, a new type of electroluminescent device, the symmetrically configured AC light-emitting (SCALE) device, is described. The mechanism for the SCALE device operation is discussed with the emphasis on the role of the interfaces.