Polythiophenes are one of the most important classes of conjugated polymers, with a wide range of applications, such as conducting films, electrochromics, and field-effect transistors, which have been the subject of a number of older and more recent reviews. Much less attention has been paid to the light-emitting properties of this class of materials, although their unique properties present a number of opportunities unavailable from more popular polymeric light emitters such as polyfluorene or poly(p-phenylene vinylene). This article reviews achievements to date in applications of thiophene-based polymers and oligomers as electroluminescent materials. We demonstrate the basic principles of controlling the optical properties of polythiophenes through structural modifications and review the most important light-emitting materials created from thiophene derivatives. Special attention is paid to consequences of structural variations on the performance of light-emitting diodes fabricated with these materials.