We describe a fuel monitoring system, comprising: an evanescent wave fuel sensor for monitoring said fuel, the sensor comprising a light source and a transducer for providing an electrical signal in response to light from said light source, and having a light path between said light source and said transducer including an evanescent wave sensing region; and a processor coupled to said fuel sensor, for converting said electrical signal from said transducer into a signal characterising said monitored fuel; and an output coupled to said processor, for outputting a signal responsive to said fuel characterising signal. We further describe apparatus for jointly determining depth of fluid in a tank and characterising the fluid according to depth, and a fluid storage tank having a three dimensional structure for containing fluid, and a three-dimensional array of fibre-optic sensors integrated into said tank structure. Embodiments may use cavity ring-down spectroscopy.