Light-emitting π-conjugated polymers and their nanostructures have been intensively studied from the viewpoints of both fundamental research and optoelectronic applications. The characteristics of light-emitting polymer nanostructures, such as light absorption and emission efficiencies, can be tuned through chemical processing and by varying their physical dimensions. In this review article, recent progress in the synthesis, characterization, modification, and applications of light-emitting polymer-based nanostructures is presented. Various synthetic methods for light-emitting polymer nanostructures are introduced, and their intrinsic optical properties at a nanoscale level are summarized. Post-synthetic treatments for modification of the characteristics related to the morphologies and doping states are discussed. Finally, potential applications of these nanostructures to barcode/quasi-superlattice nanowires, biosensors, and nano-optoelectronics are presented.