There is increasing interest in mid-infrared (mid-IR) light emitting diodes which operate in the 2-5 mum spectral region. Efficient LEDs operating at the characteristic absorption wavelengths of target gases, such as CH4, CO2 and CO, have great potential for-the next generation of optical gas sensors. The fundamental difficulties associated with realizing suitable mid-IR LEDs at different wavelengths, with high continuous wave (CW) output power at room temperature, relate principally to quantum efficiency and optical extraction of the light. Each of these will be briefly considered. Some of the different device designs and techniques used for the suppression of non-radiative Auger recombination and the reduction of Shockley-Read-Hall centres are discussed. Liquid phase epitaxy (LPE) continues to hold a strong position in mid-IR LED technology and many of the best LEDs currently available have been fabricated using this technique. In this respect, the LPE growth of associated InAs(Sb)-based epitaxial structures and their purification is briefly reported. An overview of the 'state of the art' is also given with respect to the application of mid-IR LEDs in practical gas sensors.