Rare-earth-doped optical fibres are of much interest as fibre lasers and amplifiers. They have also been used successfully as temperature sensors, by exploiting the thermodynamic properties of the energy levels of these rare-earth ions in which the relative population of a group of closely spaced levels follows the Boltzmann ratio. This paper explains the modelling of the concentration dependence of fluorescence from Neodymium-doped Optical Fibres. It also states the application to the sensing.