Traditionally, aircraft fuel systems have incorporated various conventional electrical-mechanical sensors, which operate in the potentially explosive environment of aircraft fuel tanks. Presently, in the aerospace industry extensive research is being undertaken for future aircraft fuel systems based on photonic technology. The use of optically based sensing offers a number of advantages over conventional electrical-mechanical sensors, in such difficult environments. Such optical sensors generally eliminate the necessity for moving parts, are chemically immune to corrosive environments, are less prone to produce explosion and fire hazards (for low levels of light intensity) and provide immunity from electromagnetic interference. The chromatic optical sensors (point fuel sensors and multilevel sensors) presented in this thesis exploit the measurement technique of chromatic modulation that has been devised to overcome the effects of intensity fluctuations in an optical signal. Chromatic modulation techniques have a number of advantages over conventional modulation techniques, with regard to system simplicity, cost and optical power efficiency, in that the whole of the available light power spectrum is used. The chromatic optical sensors have been used to monitor a wide range of fuel related parameters such as discrimination between different media (air, fuel, water and foam); differentiation between different types of aviation fuels; distinction between different mixtures of water in fuel; liquid level measurement and to discriminate between layers of immiscible fluids. The chromatic methodology has not only been deployed in the optical wavelength domain but also in the spatial domain for two dimensional information compression and in the time domain for signal fluctuations quantification. The experimental results of this thesis provide a feasibility study concerning the implementation of chromatic monitoring systems, which could be incorporated for fuel quality, type and quantity measurements in aircraft fuel tanks.