Traditionally, polymers have been known for their flexibility and ease of processing, but not for their electrical properties. Then it was found that polymers could be grown in such a way that they demonstrated semi-conducting properties. This arises from overlap of electron orbitals along the polymer chain, forming a valence and a conduction band, analogous to how these form in silicon. Such materials, termed "conjugated" polymers, are extremely attractive in that they combine semiconductor properties with the advantages of processing and the mechanical properties associated with polymers. Thin-film structures can be fabricated from these polymers, including semiconductor devices such as field-effect transistors which can be prepared over large areas. Recently we have found that conjugated polymers, with improved purity, and which show very strong photoluminescence, can form the "active" or electroluminescent layer in light-emitting diodes (LEDs). Materials processing and device growing has now reached a stage where control of the colour of light emitted and levels of efficiency are acceptable for display applications, such as instrument panels and pixellated flat-screen displays.