Emission from organic materials is usually fluorescence from decay of singlet states, but in LEDs a majority of the excited states generated are triplet states which can only decay by phosphorescence or by thermally-activated delayed fluorescence (TADF). To improve the potential maximum efficiency of LEDs, it is necessary to incorporate into the emissive material chromophores which are phosphorescent or which show TADF. The ways in which such units can be incorporated into polymers are described and compared and the device results to date and prospects for future development discussed.