In the thesis light intensity attenuation-based fibre-optic chemical sensors are investigated. The investigation proceeds along two lines: theoretical analyses and design studies. Both intrinsic and extrinsic types of sensors are discussed from both aspects. The theoretical analyses are concentrated on the construction of theoretical models and numerical calculations, which were previously not well developed but have been attracting a great deal of attention of researchers for some time. Under this topic, fibre-optic evanescent wave absorption sensors and fibre-optic surface plasmon resonance (SPR) sensors are studied as examples of the intrinsic type of sensors, and two-wavelength Beer-Lambert law based fibre-optic sensors as the extrinsic example. The design studies are concentrated on the design of novel types of sensors which have potentially valuable applications. As the intrinsic type of sensors, novel types of fibre-optic surface plasmon resonance (SPR) chemical sensors are proposed and studied in both point and distributed systems. A fibre-optic sensor for concentration determination of the infusible anaesthetic Propofol is studied in detail as an example of an extrinsic type of sensor.