Abstract: A fibre optic sensor, particularly for ascertaining fluid levels, utilizes at least two optical fibres having at least a portion thereof embedded in a transparent substrate material of similar refractive index. One of the fibres is coupled to a light source, the other to a light detector. The source fibre illuminates the interior of the substrate so that light exiting the source fibre is coupled to the detector fibre when total internal reflection at the substrate/fluid interface occurs in the presence of a first fluid. In the presence of a second fluid of higher refractive index than the first fluid, there will be no coupling, due to the loss of light into the fluid by refraction. The mechanism for coupling light into the detector fibre is fluorescence within that fibre causing it to provide a light signal which varies with the level of the second fluid. Several different embodiments, to achieve continuous level sensing or the sensing of other parameters, are disclosed.